The sociological and psychological literature almost uniformly suggests that young boys experience more stress than young girls, in part because of social expectations and in part because of different rates of biological development and other physiological factors. But it also suggests that with advancing age the sex difference in experienced stress tends to disappear and by late adolescence the situation of girls appears to be at least as stressful if not more so than that of boys. On the premise that the differences in experienced stress would be reflected in rates of treated mental illness, the national rates of males and females in psychiatric treatment between ages five and nineteen are compared. The data consistently show preadolescent males to have higher rates of mental illness, while by late adolescence females appear to have as high if not higher rates of mental illness than males.