The effect of stimulants on growth has been controversial. Among hyperactive children receiving long-term methylphenidate hydrochloride treatment, we examined the effects of methylphenidate withdrawal on the growth of hyperactive children randomly assigned to be taken off, or remain on, the medication regimen over two consecutive summers. After one summer, no group difference in height was found, but weight was higher in the group that had been taken off methylphenidate therapy. In contrast, two summers of being off methylphenidate treatment had a significant positive effect on height but not on weight. The results document a linkage between exposure to methylphenidate and reduction in growth velocity. However, they do not address whether the medication has long-term effects on height.