Antisocial personality disorder (APD) is a serious public and mental health concern. Understanding how well conduct disorder (CD) and other mental disorders predict the development of APD among youths involved in the juvenile justice system is critical for prevention. The authors used a stratified random sample of 1,112 detained youths to examine the development of APD at a 3-year follow-up interview. Nearly one fifth of male juvenile detainees later developed APD; approximately one quarter of male juvenile detainees with CD at baseline later developed APD. Significantly more males than females developed APD; no differences were found by race/ethnicity. Having 5 or more symptoms of CD, dysthymia, alcohol use disorder, or generalized anxiety disorder was significantly associated with developing modified APD (M-APD; APD without the CD requirement). Some disorders were strong predictors of APD; however, none were adequate screeners for identifying which detainees would later develop M-APD. The findings of this study have implications for interventions and further research in developmental psychopathology.