This study reconsiders the relationship of childhood Attention Deficit/Hyperactivity Disorder (ADHD) and childhood conduct problems with adult criminal activity by clarifying the role of the cardinal behaviors associated with the DSM-IV ADHD subtypes (inattention and hyperactivity/impulsivity). Since their childhood (average age 9 years), 230 male and 75 female subjects have been followed prospectively and were interviewed as young adults (average age 26 at follow-up). Early childhood behavior ratings by parents and teachers were examined to determine the role of conduct problems, hyperactivity-impulsivity, and inattention in predicting adult criminal involvement as measured by both official arrest records and self-report. Results show that both hyperactivity-impulsivity and early conduct problems independently, as well as jointly, predict a greater likelihood of having an arrest record for males, but not for females. For male subjects with 10 or more self-reported crimes, both early conduct problems and hyperactivity-impulsivity were significant predictors, both alone and in combination. Therefore, it appears that predominantly the symptoms of hyperactivity-impulsivity, but not inattention, contribute to the risk for criminal involvement over and above the risk associated with early conduct problems alone.