Background: The co-occurrence of attention-deficit hyperactivity disorder (ADHD) and psychoactive substance use disorder (PSUD) in adults has been the focus of much clinical and scientific inquiry. In this study we examine the effects of ADHD on the transitions from substance abuse to dependence and between different classes of agents of abuse. Methods: An ADHD sample of 239 consecutively referred adults of both genders with a clinical diagnosis of childhood-onset and persistent DSM-III-R ADHD confirmed by structured interview were compared with 268 non-ADHD healthy adults. Results: ADHD was associated with a twofold increased risk for PSUD. ADHD subjects were significantly more likely than comparisons to make the transition from an alcohol use disorder to a drug use disorder (hazard ratio = 3.8) and were significantly more likely to continue to abuse substances following a period of dependence (hazard ratio = 4.9). Conclusions: ADHD is associated with a sequence of PSUD in which early alcohol use disorder increases the risk for subsequent drug use disorder, and early substance dependence increases the risk for subsequent substance abuse. If confirmed such developmental pathways might lead to preventive and early intervention strategies aimed at reducing the risk for PSUD in ADHD subjects.