Previous research showed that the earlier the onset of antisocial behavior, the higher the likelihood that individuals will go on to engage in further antisocial acts. This study attempted to overcome several limitations with which prior research encountered. Three primary questions were addressed in this study: (1) did groups that differed on onset timing show different patterns of serious offending; (2) was that difference attributable to onset age once other relevant predictors were considered; and (3) did this relationship vary by gender? Data was gathered from subjects participating in the Philadelphia Collaborative Perinatal Project (CPP) and combined with criminal history records from the Philadelphia Police Department to explore the relationships between gender, early onset, and the seriousness of offending. Analyses revealed that early onset, before age 14, significantly predicted a summary seriousness score by age 18 for both males and females. In conjunction with prior research, males were responsible for the majority of the criminal offenses, and males who incurred an early onset were most implicated in serious offending. The knowledge that early initiation of antisocial behavior serves as a risk factor for negative outcomes later in life has influenced intervention and prevention efforts in recent years.