Subjects were participants in the Pittsburgh Youth Study, a high-risk longitudinal survey of the causes and correlates of early forms of delinquency. The sample was randomly selected from 4th grade boys enrolled in Pittsburgh public schools. Subjects participated in a 90-minute impulsivity and cognitive testing session. Factor analysis of 12 different impulsivity measures revealed two different types of impulsivity, cognitive and behavioral. Cognitive and behavioral impulsivity had differential relations with several criterion variables. Cognitive impulsivity was more strongly associated with socioeconomic status and intelligence than was behavioral impulsivity. On the other hand, behavioral impulsivity was more strongly related to delinquency at ages 10 and 12-13 years than was cognitive impulsivity. The results also indicated that both cognitive and behavioral impulsivity types were positively and significantly associated with serious delinquency that was stable over time. Behavioral impulsivity, however, was especially important for delinquency of this kind. 88 references, 8 tables, and 2 figures