The current and lifetime comorbidity of depressive (i.e., major depressive disorder and dysthymia) with other common mental disorders was examined in community samples of older adolescents (n = 1,710) and adults (n = 2,060). Current and lifetime histories of depression in the adolescents were highly comorbid with several other mental disorders. The adults had a lower but statistically significant degree of comorbidity, primarily with substance use disorder. Depression in both groups was more likely to occur after the other disorder rather than to precede it. Comorbidity did not affect the duration or severity of depression. Comorbidity in the adolescents was associated with greater frequency of suicidal behavior and treatment seeking. The findings suggest that early-onset depression is associated with a greater degree of comorbidity and may represent a more serious form of the disorder.