In the first three months of this year, among poor people age 18 to 64, about 28 percent lacked health insurance, down from 39.3 percent in 2013, the government said. And among people in that age bracket with incomes from the poverty level up to twice that amount, 23.8 percent were uninsured in the first quarter of this year, down from 38.5 percent in 2013.

Gains in coverage were also notable for Hispanics, who have long been more likely than other groups to be uninsured. In the first quarter of this year, 28.3 percent of Hispanics age 18 to 64 were uninsured, down from 40.6 percent in 2013, the government said.

The comparable figures also fell for non-Hispanic black adults, to 15.6 percent from 24.9 percent, and for non-Hispanic white adults, to 8.7 percent from 14.5 percent.

States that expanded Medicaid have seen a sharper drop in the proportion of people who are uninsured, although residents of those states were also more likely to have coverage before the health law took effect. In states that expanded Medicaid, 10.6 percent of people age 18 to 64 were uninsured in the first quarter of this year, down from 18.4 percent in 2013, the report said.

In states that chose not to expand Medicaid, 16.8 percent of such adults were uninsured in the first quarter of this year, down from 22.7 percent in 2013.

A separate study, issued this week by the Gallup organization, found that Texas was the only state where at least 20 percent of people were uninsured. By contrast, it said, in 2013, people without coverage accounted for at least 20 percent of the population in 14 states. Arkansas, California and Kentucky were among the states showing the largest reductions in the proportion of people without insurance.