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I have Medicare and a job plan. Which one pays first, and what if the wrong one was billed?

Health insuranceWho paysChecked Nov 6, 2026

From K.Actuary, founded by Sangmin Kwon, ASA · sources checked 2026-11-06 · Source: Medicare.gov · How Medicare works with other insurance · page text View original ↗

In short

  1. Two plans means one pays first and one pays the rest.
  2. Who is first depends on age, job, plan size and illness.
  3. For an aged worker, a plan from an employer with 20 or more employees can come first.

What this means

Medicare.gov says each kind of coverage you have is a "payer." The primary payer pays up to the limits of its coverage. Then it sends the rest of the balance to the secondary payer [1]. If the secondary payer doesn't cover the rest, you may owe it [1]. This order is called coordination of benefits [1]. CMS says these rules decide which one pays first [2].

Who pays first depends on your case. For a person the rule calls "aged," Medicare is secondary when two things are true. The person is covered by a group health plan from an employer with at least 20 employees, and the coverage comes from current employment [3]. A spouse's current job can count too [3]. If you refuse the employer plan, Medicare is primary for you [3]. For a person entitled to Medicare Part A under § 406.12 of the rule, Medicare is secondary to a large group health plan when the coverage comes from their own or a family member's current employment [4]. A large plan means one whose employer has at least 100 employees [5]. For kidney failure (ESRD), Medicare is secondary to a group plan during a set coordination period [6].

The sources here do not say who pays first for retiree coverage alone, or for other kinds of plans. Medicare.gov says that if your group plan or retiree plan is secondary, you may need to sign up for Part B before it pays [1]. If the insurer does not pay promptly, usually within 120 days, your doctor may bill Medicare. Medicare may make a conditional payment and later recover what the primary payer should have paid [1].

Your next step

Check the two conditions in Source 3 for an "aged" person, and the large-plan size in Source 5 [3] [5]. The time these sources give is usually 120 days before your doctor may bill Medicare if the insurer has not paid [1].

Check it yourself

Look at your group plan's papers for the employer's size and how your coverage is tied to a job. Open Source 3 and read the two conditions for an "aged" person. "Don't know yet" for your own plan.

Where this comes from (6)

  1. [1] Medicare.gov · How Medicare works with other insurance · page text · checked 2026-11-06

    each type of coverage is called a "payer." … The "primary payer" pays up to the limits of its coverage, then sends the rest of the balance to the "secondary payer." … If the "secondary payer" doesn't cover the remaining balance, you may be responsible for the rest of the costs … If your group health plan or retiree coverage is the secondary payer, you may need to sign up for Medicare Part B before they'll pay … This …

  2. [2] CMS · Coordination of Benefits & Recovery Overview · page text · checked 2026-11-06

    If a beneficiary has Medicare and other health insurance, Coordination of Benefits (COB) rules decide which entity pays first

  3. [3] GovInfo · 42 CFR § 411.172 Medicare benefits secondary to group health plan benefits · (a) and (c) · checked 2026-11-06

    Medicare Part A and Part B benefits are secondary to benefits payable by a GHP for services furnished during any month in which the individual- (1) Is aged; … Is covered under a GHP of an employer that has at least 20 employees (including a multi-employer plan in which at least one of the participating employers meets that condition), and coverage under the plan is by virtue of the individual's current employment sta…

  4. [4] GovInfo · 42 CFR § 411.204 Medicare benefits secondary to LGHP benefits · (a) · checked 2026-11-06

    Medicare benefits are secondary to benefits payable by an LGHP for services furnished during any month in which the individual- (1) Is entitled to Medicare Part A benefits under § 406.12 of this chapter; (2) Is covered under an LGHP; and (3) Has LGHP coverage by virtue of his or her own or a family member's current employment status.

  5. [5] GovInfo · 42 CFR § 411.101 Definitions · Large group health plan (LGHP) · checked 2026-11-06

    Large group health plan (LGHP) means a GHP that covers employees of either- (1) A single employer or employee organization that employed at least 100 full-time or part-time employees

  6. [6] GovInfo · 42 CFR § 411.162 Medicare benefits secondary to group health plan benefits · (a)(1) · checked 2026-11-06

    Medicare is secondary to any GHP (including a retirement plan), with respect to benefits that are payable to an individual who is entitled to Medicare on the basis of ESRD, for services furnished during any coordination period determined in accordance with paragraphs (b) and (c) of this section

How we found this

We matched your question to 6 official sources, quoted the exact lines, and re-checked each quote against its source on 2026-11-06.

What we don’t know yet

Your own plan or notice may say something different. If your letter cites another rule, send it and we will look again.

Every answer is checked against the official rule it quotes.

Information only — not legal advice. Your plan, insurer or agency decides.

This answer quotes the official sources listed above, with the date we checked them. Your own policy, notice and state rules control. We are not your lawyer or your insurer. We don’t file, argue or negotiate claims for you — you send any letter or form yourself.

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