American children are gaining weight at an alarming rate. Since the 1960s, according to the Centers for Disease Control and Prevention (CDC), the percentage of American six- to eleven-year-olds who fall into the CDC's highest weight classification for children has almost quadrupled. Requiring more physical education (PE) seems like a logical response to the childhood obesity epidemic, but will mandating more time in gym classes actually result in more exercise for kids? Will it help them lose weight? PE classes may generate an increase in exercise at the moment the class is held, but students may decide to exercise less at other times during the school week. And even if overall exercise levels jump upward, that will not lead to weight loss if students increase their caloric consumption. Evaluations of innovative PE curricula designed to encourage exercise suggest that PE classes do increase physical activity but have no noticeable effect on student weight. Still, relatively little research has systematically examined how much PE contributes to weight loss or lowers the risk of obesity, and what little research there is finds no association between PE and weight loss and obesity. To investigate the matter further, the authors examined how differences in state requirements for PE affect the amount of time students spend exercising in PE class. Then they looked at how much that increase in PE exercise time affects the levels of overall physical activity and the weight of high-school students. Their results in a nutshell: when states raise their PE requirements, girls become more active, as indicated by their reports of the total number of minutes per week they say they are exercising vigorously. There is no similar effect on boys. When it comes to less-vigorous physical activity, however, increased PE exercise actually "decreases" the number of days in which girls report light physical activity. Apparently, when girls exercise in class, they become more sedentary during the discretionary hours of their week. Unfortunately, this propensity occurs predominantly among girls who are less active in the first place. Perhaps because PE has so little impact on physical activity, the authors found little effect on weight loss or the likelihood of obesity, as measured by the body mass index (BMI). The only suggestion of an effect is among the most-sedentary girls, and the evidence there is weak at best. (Contains 2 figures.)