OBJECTIVE: This study investigated the early characteristics of children who at 8 years old were hyperactive, aggressive, or both, to clarify the etiology of these disorders. METHOD: Prospective data from infancy to 8 years on children who were hyperactive (N = 65), aggressive (N = 57), or both (N = 60) at 8 years were compared with those on a normal comparison group (N = 70). Variables included ratings of temperament, behavior, school performance, and socioeconomic and life stress indices. RESULTS: The two aggressive groups, particularly the hyperactive-aggressive group, were more difficult in temperament and behavior from infancy and had less optimal environments. In contrast, the "pure" hyperactive group showed more problems than did the comparison group only from 3 to 4 years on. All three clinical groups had poorer academic performance than did the controls. CONCLUSIONS: The findings are consistent with a transactional model of development in which aggression (with or without hyperactivity) emerges when difficultness in infancy interacts with a stressed environment. Hyperactivity, when unassociated with aggression, may emerge later from poor self-regulation when faced with societal, especially school, demands. The pattern of group differences found suggest that risk indicators for specific patterns of later maladjustment may be identifiable for early intervention.