Objective: To examine the influence of conduct disorder (CD) on substance use initiation. Method: Community adolescents without CD (n = 1,165, mean baseline age = 14.6 years), with CD (n = 194, mean baseline age = 15.3 years), and youth with CD recruited from treatment (n = 268, mean baseline age = 15.7 years) were prospectively followed and re-interviewed during young adulthood (mean ages at follow-up respectively: 20, 20.8, and 24). Young adult retrospective reports of age of substance initiation for 10 substance classes were analyzed using Cox regression analyses. Hazard ratios of initiation for the CD cohorts (community without CD as the reference) at ages 15, 18, and 21 were calculated, adjusting for baseline age, gender, and race/ethnicity. Results: Among community subjects, CD was associated with elevated adjusted hazards for initiation of all substances, with comparatively greater hazard ratios of initiating illicit substances at age 15 years. By age 18, the adjusted hazard ratios remained significant except for alcohol. At age 21, the adjusted hazard ratios were significant only for cocaine, amphetamines, inhalants, and club drugs. A substantial portion of community subjects without CD never initiated illicit substance use. Clinical youth with CD demonstrated similar patterns, with comparatively larger adjusted hazard ratios. Conclusions: CD confers increased risk for substance use initiation across all substance classes at age 15 years, with greater relative risk for illicit substances compared to licit substances. This effect continues until age 18 years, with the weakest effect for alcohol. It further diminishes for other substances by age 21, However, the likelihood of initiating cocaine, amphetamines, inhalants and club drug use among those who have not initiated yet continues to be highly elevated by age 21. (Contains 4 tables and 4 figures.)