The Mississippi Medicaid program has been erroneously paying the Medicare Part B premiums for about 9,300 people who are no longer on Medicaid.
Officials are still trying to determine how much money the mistake has cost the state.
Medicaid spokesman Francis Rullan said Thursday that a software glitch was discovered in the past week, and the state government told the federal government about the problem.
Rullan said the problem dates as far back as 2003 and involves cases in which Medicaid paid the Part B Medicare insurance premium. He blames the error on a private company that handles Medicaid billing.
Rullan said the recipients will have to start paying Part B premiums of about $96 a month. The recipients are being notified in letters sent by the federal government.
Once officials determine how much money the state lost, Medicaid will try to recover the money from the billing company.
Medicaid is paid by a combination of state and federal money. Medicare is federal only.