Deviant peer group affiliation was evaluated as a risk factor for substance use in adolescents with childhood attention-deficit/hyperactivity disorder (ADHD). Results showed that deviant peer affiliation mediated the relationship between ADHD and substance use, suggesting that children with ADHD are more likely than children without ADHD to become involved with deviant peers and, as a result, more likely to use substances. Moreover, the relationship between deviant peer affiliation and substance use was stronger for adolescents with ADHD, suggesting that once they are immersed in a deviant peer group, adolescents with ADHD are more vulnerable to the negative social influences of that group. This study is the first step in identifying high-risk pathways from childhood ADHD to substance use in adolescence.