Life expectancy has long been growing steadily for most Americans. But it has not for a significant minority, according to a new study, which finds a growing disparity in mortality depending on race, income and geography.

The study, published Monday in the online journal PLoS, analyzed life expectancy in all 3,141 counties in the United States from 1961 to 1999, the latest year for which complete data have been released by the National Center for Health Statistics. Although life span has generally increased since 1961, the authors reported, it began to level off or even decline in the 1980s for 4 percent of men and 19 percent of women.

“It’s very troubling that there are parts of the wealthiest country in the world, with the highest health spending in the world, where health is getting worse,” said Majid Ezzati, the lead author and an associate professor of international health at Harvard. It is a phenomenon, he added, “unheard of in any other developed country.”

Counties with significant declines were concentrated in Appalachia, the Southeast, Texas, the southern Midwest and along the Mississippi River. Life expectancy increases were mainly in the Northeast and on the Pacific Coast.