About 112,00 Michigan residents chose a private insurance plan under the federal health care law through January, outpacing what was projected in a government memo last summer.
Roughly 36,000 people picked a plan last month, the fourth month of enrollment that ends in late March.
Eighty-six percent of all those signing up have qualified for tax credits to offset a portion of their premium.
A memo to Health and Human Services Secretary Kathleen Sebelius projected Michigan would have 100,000 signups through January.
The Obama administration is unable to say how of many of those enrolling for coverage previously were uninsured. Some might have been among residents whose previous policies were at risk of being canceled because they didn't meet the law's standards.