How Medicare claims work and who files them
In most cases, you do not file a Medicare claim yourself. When you see a doctor, hospital, or other provider who accepts Medicare, that provider sends the claim to Medicare on your behalf. Medicare processes it, decides what portion it will pay, and sends you an Explanation of Benefits (EOB) — a document that shows what was billed, what Medicare paid, and what you owe.
However, there are situations where you may need to file a claim yourself: if you paid out of pocket for a service and want to seek reimbursement, if a provider does not accept Medicare and you paid them directly, or if a claim was denied and you want to appeal it. Understanding when and how to file is the difference between getting reimbursed and losing money.
The process differs slightly depending on whether you have Original Medicare (Parts A and B) or a Medicare Advantage plan (Part C). This guide covers Original Medicare, which is what most people use.
Key Takeaways
- Most providers file claims for you automatically, so you receive an Explanation of Benefits in the mail showing what Medicare paid and what you owe.
- You file a claim yourself only if you paid out of pocket, saw a provider who does not accept Medicare, or need to appeal a denial.
- Original Medicare claims go to your regional Medicare Administrative Contractor (MAC), which you can find by entering your ZIP code at cms.gov/mac.
- Paper claims use Form CMS-1500 for doctors and Form CMS-1450 for hospitals; you can also file electronically through your provider's office or a billing service.
- Keep receipts, the provider's invoice, and any proof of payment for at least three years in case Medicare asks questions.
When you need to file a claim yourself
You typically file a claim when you have already paid a provider and want Medicare to reimburse you. This happens most often when you see a provider who does not accept Medicare assignment — meaning they do not agree to accept Medicare's payment as full payment for the service. You pay them the full amount, then file a claim asking Medicare to send its portion directly to you.
Another common situation is when you travel out of state or out of the country and receive emergency care. If the provider does not have a relationship with Medicare, you pay first and file a claim later. You may also file a claim if you paid for a service you thought Medicare would not cover, then later learned it would have.
If you have a Medicare Advantage plan instead of Original Medicare, contact your plan directly — they handle claims differently and may have their own filing process.
Finding your Medicare Administrative Contractor and gathering documents
Your claim goes to a Medicare Administrative Contractor (MAC), a private company that processes Medicare claims for your region. To find yours, visit cms.gov/mac and enter your ZIP code. The website will show you which MAC serves your area and provide their mailing address, phone number, and sometimes an online portal.
Before you file, gather these documents: the itemized bill or invoice from the provider showing the date of service, what was done, and the charge; your receipt or proof of payment (credit card statement, cancelled check, or receipt from the provider); your Medicare card; and any documentation that explains why you paid out of pocket (for example, a letter from the provider saying they do not accept Medicare).
If the service was denied by Medicare initially, also collect the denial letter or EOB that explains why. This helps the MAC understand your situation when you file.
Filing a paper claim using Form CMS-1500
For doctor visits, lab work, and outpatient services, you use Form CMS-1500. You can read this form from cms.gov by searching "CMS-1500" or request it from your MAC. The form has 33 numbered boxes, and each one asks for specific information.
Fill in your name, address, and Medicare number in the top section. In the service section, list the date of service, what was done (using the provider's procedure code if you have it, or a description), the charge, and the amount you paid. Include the provider's name, address, and National Provider Identifier (NPI) number — you can find the NPI by searching the provider's name at npi-registry.cms.hhs.gov.
Sign and date the form, then mail it to your MAC along with copies (not originals) of your receipt and the itemized bill. Keep a copy for your records. Processing typically takes four to six weeks, though it can vary.
Filing a paper claim for hospital services using Form CMS-1450
Hospital claims use a different form: Form CMS-1450, also called the UB-04. This form is more complex than the CMS-1500 because hospital bills include many line items — room charges, surgery, lab work, medications, and more. Most hospitals file this form for you automatically, but if you need to file it yourself, ask the hospital's billing department for a copy of the form and instructions specific to your bill.
The CMS-1450 is rarely filed by patients; hospitals and billing services handle it. If you are unsure whether you need this form, call your MAC and describe the service. They can tell you which form applies.
Filing claims electronically or through a billing service
If you have a computer and internet access, you can file electronically through your MAC's online portal. Visit your MAC's website (found at cms.gov/mac) and look for a patient portal or online claim submission option. You will need to create an account and upload images of your documents.
Alternatively, you can ask your provider's billing office to file the claim for you, even if they did not file it initially. Many offices will do this at no charge. If the provider refuses or is out of business, you can hire a medical billing service or a patient advocate to file on your behalf — these services typically charge a percentage of what Medicare reimburses, usually 10 to 15 percent.
Electronic filing is faster than paper mail and gives you a confirmation number when ready. If you are comfortable uploading documents, it is worth trying your MAC's online option first.
Understanding your Explanation of Benefits and what happens next
After Medicare processes your claim, you will receive an Explanation of Benefits (EOB) in the mail. This document shows what the provider billed, what Medicare determined is the allowable amount, what Medicare paid, and what you owe. Read it carefully to make sure the dates, services, and amounts match what you remember.
If Medicare approved the claim and paid, the money goes to you (if you filed) or to the provider (if they filed). If Medicare denied the claim or paid less than you expected, the EOB will explain why. Common reasons include: the service is not covered by Medicare, the provider is not Medicare-enrolled, or the service was deemed not medically necessary.
You have the right to appeal a denial. The EOB includes instructions for filing an appeal, and you have 120 days from the date on the EOB to start the process. Keep your EOB and all related documents for at least three years.
Frequently Asked Questions
What if I file a claim and Medicare says the provider should have filed it?
This sometimes happens when a provider accepts Medicare but did not submit the claim. Call your MAC and explain that the provider did not file. Medicare may contact the provider directly and ask them to file, or Medicare may process your claim and reimburse you. Either way, do not pay the provider again.
How long does it take Medicare to process a claim?
Paper claims typically take four to six weeks. Electronic claims are often processed faster, sometimes within two to three weeks. If you do not receive an EOB within eight weeks, call your MAC to check the status.
Can I file a claim for a service Medicare already denied?
Yes, but filing the same claim again will likely result in the same denial. Instead, file a formal appeal using the instructions on your denial letter. An appeal gives you a chance to provide additional information or documentation that might change the outcome.
What if the provider charges more than Medicare allows?
If the provider accepts Medicare assignment, they cannot charge you more than Medicare's allowable amount plus your copay or coinsurance. If they do not accept assignment, they can charge more, but Medicare will only reimburse based on the allowable amount — you are responsible for the difference.
Do I need to file a claim if I have a Medigap or Medicaid plan?
No. If you have Medigap (supplemental insurance), Medicare files first, then your Medigap plan receives the claim automatically and pays its portion. If you have Medicaid, Medicaid coordinates with Medicare. In both cases, the plans handle filing — you do not need to do anything.