The purpose of this study was to identify group differences in children with attention-deficit-hyperactivity disorder and motor dysfunction (ADHD-MD) and ADHD only, and to evaluate the medication responsiveness of ADHD-MD. Sixty-three children (49 males and 14 females; mean age 9 years 10 months, SD 2 years 10 months) underwent a triple blind, placebo-controlled crossover study evaluating two dose levels of methylphenidate (0.3 mg/kg and 0.05 mg/kg, twice daily) and placebo. Forty-nine trials were completed. Nineteen were children with ADHD-MD, 44 had ADHD only. Behavior and functioning were assessed at home and at school. Treatment effects were assessed using the Abbreviated Symptom Questionnaire for Parents and Teachers. Children with ADHD-MD were more likely to have severe ADHD-combined type and other neurodevelopmental and behavioral problems. Both groups of children had a linear dose response to medication (placebo, low, high) and there was no evidence of a group by dose interaction or an overall group effect at home or school. The lack of group effect suggests that these children responded to medication like the other subgroups.