One in three American teens gets an "F" in physical fitness.

The finding comes from a national sample of 3,287 kids aged 12-19. The study included a treadmill test. The shocking result: A third of U.S. boys and girls fail to meet acceptable fitness standards.

"This represents a significant public health problem because low physical fitness during adolescence tends to track into adulthood, and low-fit adults are at substantially increased risk for [illness and death]," conclude University of South Carolina exercise scientist Russell R. Pate, PhD, and colleagues.

Their report, in the October issue of Archives of Pediatrics, may herald worse news to come. The least physically fit group in the study was boys aged 12-13. Nearly half of them -- 44.7 percent -- failed to meet fitness standards.

The fitness standard used in the study is a measure of the body's ability to use oxygen during exercise. When people are physically unfit, they can't use oxygen efficiently and can't endure much exercise.

The findings hold true regardless of kids' race or ethnicity. As might be expected, the least-fit kids tend to be the heaviest, least-active kids.

Older boys were more fit than younger boys, but older girls tended to be less fit than younger girls.

Tool: Use WebMD's Kids Fitness Finder

By Daniel J. DeNoon, reviewed by Louise Chang, MD