The present study sought to examine subtype differences in comorbidity and in antisocial, educational, and treatment histories among young adults (ages 17-27) with attention deficit hyperactivity disorder (ADHD). Comparisons were made between ADHD Combined Type (ADHD-C; N = 60) and Predominantly Inattentive Type (ADHD-I; N = 36) relative to each other and to a community control group of 64 adults. Both ADHD groups had significantly less education, were less likely to have graduated from college, and were more likely to have received special educational placement in high school. Both groups also presented with a greater likelihood of dysthymia, alcohol dependence/abuse, cannabis dependence/abuse, and learning disorders, as well as greater psychological distress on all scales of the SCL-90-R than the control group. Both ADHD groups were more likely to have received psychiatric medication and other mental health services than control adults. In comparison with ADHD-I, adults with ADHD-C differed in only a few respects. The C-type adults were more likely to have oppositional defiant disorder, to experience interpersonal hostility and paranoia, to have attempted suicide, and to have been arrested than the ADHD-I adults. These results are generally consistent with previous studies of ADHD in children, extend these findings to adults with ADHD, and suggest that the greater impulsivity associated with the ADHD-C subtype may predispose toward greater antisocial behavior and its consequences than does ADHD-I type in adults.