Twenty-six hyperactive children receiving methylphenidate hydrochloride, eight hyperactive non-medicated children and 25 normal children participated in a study of the effect of such medication on stature of hyperactive children. All the children were below 13 years of age and all but two were male. Predicted adult stature before methylphenidate treatment and predicted adult stature after one year, two years and three years of treatment were calculated by means of the Roche-Weiner-Thissen Adult Stature Prediction Formula. Total milligrams per day dosage, milligrams per kilogram per day dosage and duration of treatment per year were analysed in the medicated hyperactive children and their growth was compared with the other two groups. No significant differences were found for any of the above variables. This study indicates that stature suppression does not occur in male children below 13 years of age when methylphenidate is used up to 0.8mg/kg/day for one or two years and up to 0.6mg/kg/day for three years.