The effects of methylphenidate dosages were assessed in 42 hyperactive children who were tested off drug at the beginning and end of an 18-week study and 2 weeks after initiation of various dosages. Initial methylphenidate doses of 0.2 mg/kg were increased biweekly until 0.8 mg/kg was reached or the child showed adverse personality or physiologic effects. Measures included an objective test of the ability to sustain attention and behavioral ratings completed by parents and teachers. Data were analyzed separately for groups attaining low, moderate, and high maximum doses (0.4, 0.6, and 0.8 mg/kg, respectively). For all dosage groups behavior and attention improved with increased dosage, and the best scores were associated with the highest drug levels. In contrast to previous reports, differences in optimal dosage levels for behavior and cognitive performance were not seen. This discrepancy may result from the lack of individual titration based on tolerance in earlier reports.