The adverse side effects of methylphenidate were evaluated in 27 children with attention deficit hyperactivity disorder and IQs of 48 to 74 who participated in a double-blind study of two doses of methylphenidate and placebo. A checklist of 13 side effects, generated from the Physician's Desk Reference, was completed by teachers. Rates of irritability, anxiety, moodiness, and activity level decreased significantly when comparing the placebo with drug conditions. However, medication for six (22%) of the children was discontinued because of the appearance of motor tics (three children) and severe social withdrawal (two children), suggesting that mentally retarded children with attention deficit hyperactivity disorder may be at a greater risk for developing these side effects than the nonretarded population.