Evaluated efficacy of social skills training (SST) on children with 2 subtypes of attention deficit hyperactivity disorder (ADHD). Participants were 120 children (30 girls, 90 boys), ages 8 to 12 with ADHD-Inattentive type (ADHD-I; n = 59) or Combined type (ADHD-C; n = 61). The children were randomly assigned within diagnosis subtype to the treatment condition (8 weeks of SST) or the no-intervention control condition. SST led to greater improvements in both parent- and child-perceived assertion skills in the children with ADHD, yet did not affect the other domains of social competence. Diagnostically heterogeneous groups led to greater improvements on parent-report of their child's cooperation and assertion abilities as well as children's report of their own empathy skills. Diagnostically homogeneous groups led to greater decreases in externalizing behaviors at posttreatment but not at follow-up. Children with comorbid oppositional defiant disorder (ODD) did not benefit as much from the intervention. Children with ADHD-I improved in assertion skills more than children with ADHD-C, yet the 2 diagnostic entities did not differ in improvement levels across all other social skills.