“It’s an uptick in mortality and that doesn’t usually happen, so it’s significant,” said Robert Anderson, the chief of mortality statistics at the National Center for Health Statistics, part of the Centers for Disease Control and Prevention. “But the question is, what does it mean? We really need more data to know. If we start looking at 2016 and we see another rise, we’ll be a lot more concerned.”

The death rate rose to 729.5 deaths per 100,000 people in 2015, up from 723.2 in 2014, according to the National Center for Health Statistics. It was one of the few times in the past 25 years that the rate has increased. A bad flu season pushed it up in 2005, and AIDS and the flu contributed to a sharp increase in 1993. In 1999, there was a tiny increase.

Experts said the current rise was surprising.

“We are not accustomed to seeing death rates increase on a national scale,” said Andrew Fenelon, a researcher at the C.D.C. who did not work on the paper. “We’ve seen increases in mortality for some groups, but it is quite rare to see it for the whole population.”

He added that it would drag the United States further behind its European peers: “Many countries in Europe are witnessing declines in mortality, so the gap between the U.S. and other countries is growing.”