The study found that children with ADHD and a co-occurring externalizing disorder (mostly oppositional defiant disorder) had significantly worse drug-use outcomes (drug use frequency and substance-use disorders) compared to those individuals with ADHD without an externalizing disorder and those without ADHD (normal control group). The drugs most often used were marijuana and tobacco. Those with ADHD but without an externalizing disorder showed drug-use outcomes comparable to the community control group. The authors advise, however, that ADHD is seldom without some co-occurring behavioral disorder. It is important to have early detection of problem behavior, such as oppositional defiant disorder, so as to provide intervention that will prevent later problems with substance abuse. Data for this study were collected as part of the Minnesota Competence Enhancement Project, a longitudinal study of ADHD that began in 1991. The project identified samples of disruptive and control (nondisruptive) children drawn from 22 suburban elementary schools. The final sample consisted of 82 children with ADHD and an externalizing disorder, 27 children with ADHD only, and 91 children without ADHD. When the total sample of participants was in late adolescence, they and their parents completed an assessment battery that focused on each youth's drug-use behavior and also on their psychopathology and psychosocial adjustment. 3 tables, 1 figure, and 44 references