A pilot comparison of the safety and efficacy of methylphenidate (MPH) combined with clonidine, clonidine monotherapy, or MPH monotherapy in 6- to 16-year-old children diagnosed with attention deficit hyperactivity disorder (ADHD) and comorbid aggressive oppositional defiant disorder or conduct disorder was completed. Study design was a 3-month, randomized, blinded, group comparison with eight subjects per group. No placebo comparison was used. All three treatment groups showed significant improvements in attention deficits, impulsivity, oppositional, and conduct disordered symptoms as assessed by parent and teacher rating scales and laboratory measures. Significant differences among treatment groups were found only on a few measures. Only the clonidine monotherapy group showed significantly decreased fine motor speed. These results suggest the safety and efficacy of clonidine alone or in combination with MPH for the treatment of ADHD and aggressive oppositional and conduct disorders.