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.Attention-deficit/hyperactivity disorder (ADHD) is a persistent and pervasive mental health problem that first appears in childhood and comprises two primary symptom domains: (a) inattention and (b) impulsivity-hyperactivity (American Psychiatric Association, 1994). Several decades of research have shown that clinically significant levels of ADHD symptoms interfere with academic and social functioning (Barkley, 1998). Indeed, children with ADHD-especially boys, who are six to nine times more likely than girls to have ADHD in clinic-referred settings-experience impairment in many areas of functioning that may cause significant adjustment problems later in life. For example, a recent review concluded that childhood ADHD continues to be a risk factor for academic, behavioral, and social functioning difficulties in adolescence and young adulthood (see Mannuzza & Klein, 1999).Within the domain of possible adverse behavioral outcomes for children with ADHD is early use of drugs and alcohol. Although there is controversy surrounding the magnitude and cause of the effect, children who are clinic referred for ADHD appear to be at risk for early substance use. For example, adolescents with childhood hyperactivity more often reported cigarette, alcohol, and marijuana use than did control participants (Barkley, Fischer, Edelbrock, & Smallish, 1990), although the alcohol and marijuana group differences were Gittelman, Mannuzza, Shenker, and Bonagura (1985) reported more drug use disorders in their late adolescent males with childhood hyperactivity than in boys without childhood hyperactivity. With some exceptions for tobacco (Burke, Loeber, & Lahey, 2001;Milberger et al., 1997), substance use by adolescents with childhood ADHD is strongly associated with the development of conduct problems in these same individuals (Barkley et al., 1990;Gittelman et al., 1985). This result is not surprising given the robust, although imperfect, association between the two behavioral domains of substance use and conduct problems (Mason & Windle, 2002). The important, potentially mediating role of conduct problems is also consistent with theoretical models of early-onset alcoholism common among males (Sher, 1991;Zucker, Ellis, & Fitzge...