Efficacy of methylphenidate among mentally retarded children with attention deficit hyperactivity disorder. Twelve children with IQ scores of 50 to 74 (educable mental retardation) who met rigorous diagnostic criteria for attention deficit hyperactivity disorder participated in a double-blind crossover study of the efficacy of two doses of methylphenidate compared with placebo. Dependent measures included behavioral ratings, classroom work output, laboratory measures of attention and learning, and direct observations of social behavior. Improvement with medication on the Conners Hyperactivity Index was observed in 75% of subjects. Significant increases in work output, on-task behavior, and attentional skills were associated with methylphenidate. However, gains in measures of attention were not associated with improvement in learning, as measured by a paired associate learning task. Additionally, no significant increases in appropriate social interactions during free play were associated with methylphenidate. The results suggest that mentally retarded children with attention deficit hyperactivity disorder respond to methylphenidate at similar rates and in similar domains to that of the nonretarded population. 