While that finding is not surprising - previous research shows about 16 percent of U.S. schoolchildren are seriously overweight - it's ''very concerning,'' said David Ludwig, director of the obesity program at Children's Hospital Boston.

Ludwig, who was not involved in the study, called treadmill tests a good measure of fitness. He said the results show that ''at a time in life when adolescents and young adults should be at peak levels of fitness, there's in fact a very high prevalence . . . of very low fitness.''

The analysis of nationally representative data from government health surveys by Northwestern University researchers found that 34 percent of girls and boys ages 12 to 19 showed a poor level of cardiovascular fitness on an 8-minute treadmill test.

The tests became faster and steeper after a 2-minute warmup, and a rapidly increasing heart rate after just a short period of exercising defined poor fitness.

The study included 2,205 adolescents and 3,110 adults ages 20 to 49 who participated in the 1999-2002 surveys.

About 14 percent of the adults showed a poor level of fitness, but that underestimates the true number who are unfit because adults with high blood pressure and other cardiovascular risk factors were excluded from the treadmill test, said lead author Mercedes Carnethon.

Teens and adults with poor fitness were two to four times more likely to be overweight or obese than those considered moderately or highly fit, the study found. Waist size, cholesterol levels and blood pressure also were higher in those in the low fitness category.

The study appears in Wednesday's Journal of the American Medical Association.

''We've known for a long time that there's a trend toward declining physical activity'' among U.S. kids, said Carnethon, an assistant professor of preventive medicine at Northwestern. The study shows this ''is now being reflected by the prevalence of poor fitness.''

Average cholesterol levels were about 10 points higher for youngsters in the low fitness category than for those considered highly fit. Blood pressure differences between categories was not as dramatic. About 4 percent of the girls and almost 2 percent of the boys already had high blood pressure.

''While adolescents aren't at risk for heart disease in the short term,'' Carnethon said, ''this has important implications for the long term health of youth in the United States.''

Researchers did not calculate how many youngsters were in the moderately and highly fit categories, Carnethon said.