Moffitt's theory regarding two types of adolescent antisocial behavior was investigated using a prospective, longitudinal study of normal and abnormal development in a primarily low socioeconomic status, ethnically diverse sample. Results supported the presence of an early-onset/persistent (EOP) group and an adolescence-onset (AO) group. Groups were most reliably and significantly distinguished by indices of socioemotional history within the first 3 years, but no significant differences were found on early measures of temperament or neuropsychological functioning. EOPs scored significantly lower than other groups on measures of neuropsychological functioning only during late childhood and adolescence, suggesting that the declines in verbal functioning that have been so reliably found in this and other samples of early-starting antisocial adolescents are progressive and consequent to adverse experience. In adolescence, AOs were significantly more likely to report high levels of internalizing symptoms and life stress, suggesting that AO antisocial behavior is not a benign phenomenon. Implications of these findings for etiologic theories of adolescent antisocial behavior are discussed.