The response to methylphenidate was examined in 36 boys, aged 7-11 y, with attention-deficit hyperactivity disorder (ADHD) in a double-blind, placebo-controlled, crossover design. Hyperactivity and conduct problems were significantly reduced during methylphenidate treatment. Stimulant medication was associated with improvements on tests of sustained attention, working memory and motor steadiness. When individual changes were studied, it was found that 83% showed a significant improvement in their hyperactivity at home or at school, and for 60% their levels of hyperactive behaviour were within the normal range. High levels of hyperactivity at school and relatively low age were significant predictors of normalization of hyperactivity in at least one setting. However, these predictors could only classify correctly 71% of the children. In clinical practice a trial with stimulants is indicated in ADHD children who show symptoms that are sufficiently severe to cause impairment at home and at school.