A new study shows teenage girls are not getting enough exercise.
The report published in Thursday's New England Journal of Medicine says the amount of regular exercise girls get drops drastically when they hit their teens.
Some cases practically falls to zero, specifically blacks teens.
Researchers found, by the time the girls are sixteen or seventeen, more than half of the black girls in the study and nearly a third of the white girls reported they didn't exercise at all outside of school.
The girls' decline in physical activity was affected by lower levels of parental education, heavier weight, smoking and pregnancy.
Experts are concerned with the findings, especially with obesity at epidemic levels.
There's no similar study for boys.
But experts find they are generally more active because they participate more in sports.