This investigation addresses the mediational role of friends' characteristics between adolescents' activity breadth (i.e., variety in activity participation) and their later adjustment. Data were drawn from 2 longitudinal studies: the Childhood and Beyond (CAB; N = 925) study and the Maryland Adolescent Development in Context Study (MADICS; N = 1,338). Adolescents at Time 2 in each study (8th, 9th, and 11th graders in CAB and 8th graders in MADICS) reported on the breadth of their participation across 5 activity settings: sports, religious, volunteering, community, and school. Friends' characteristics and adolescent adjustment were reported by adolescents at Times 1 and 3. Friends' positive characteristics significantly mediated relations between activity breadth and adolescent depressive affect, self-worth, alcohol use, and problem behavior in both studies. Friends' negative characteristics significantly mediated these relations in CAB, but not in MADICS. (Contains 1 figure, 4 tables, and 2 footnotes.)