Washington ? A third of the nation’s workers without health insurance are employed by large companies, a study says.

Thirty-two percent of all uninsured workers in 2001 were employed by big companies, up from 25 percent in 1987, according to the report released Tuesday by The Commonwealth Fund.

Researchers cited as factors soaring health care costs, declines in manufacturing and union jobs and the changing structure of large corporations — those with more than 500 employees — and the benefits they offer. The Census Bureau estimates that 44 million Americans were uninsured last year.

The percentage of uninsured employees in small businesses — those with fewer than 100 workers — dropped from 67 percent in 1987 to 57 percent in 2001. Similarly, the number of uninsured workers at medium-sized companies fell from 14 percent to 12 percent during the same period.