Objective: To assess whether the relationship of an ADHD diagnosis by adolescence to nonprescription stimulant use in adulthood is direct or indirect, via Conduct Disorder (CD) and/or Substance Use Disorder (SUD). Method: Data were obtained from multiple waves of interviews and questionnaires completed by 551 community-based participants when they were between the mean ages of 14.1 and 36.6 years. Results: The results of the structural equation model (SEM) supported both a direct association between early ADHD and later nonprescription stimulant use (B = .18, z = 2.74) and the relationship from ADHD to later nonprescription stimulant use (B = .01, z = 1.72) via CD and SUD. Conclusion: The longitudinal data supporting these paths suggest that efforts to prevent and treat the misuse of nonprescription stimulants may be more effective if attention is paid to those with a history of ADHD, as well as to those who also had CD and SUD.