The authors conducted a three-week controlled, double-blind (methylphenidate-placebo) study of 31 hyperkinetic boys. Response to treatment was judged on the basis of improvement in pretreatment and posttreatment teacher ratings. Pretreatment measures indicated a high arousal state in those children who obtained a poor clinical response to methylphenidate. Changes in indicators of arousal suggested a greater physiological effect from methylphenidate treatment in the low-arousal (good clinical response) group. The authors' data suggest that genetic factors may play a major role in the etiology of this disorder; however, they stress the need for further research.Abstract Teaser