This is a study of the growth of 72 hyperactive boys treated continuously with methylphenidate hydrochloride. Major findings were that methylphenidate produces an adverse effect on growth in height and in weight in the first year of treatment, but not in the second year; the first year height deficit is offset in the second year by a greater-than-expected growth rate. No clinical predictors of growth deficits were found; growth in height deficits are not related to total dosage or summer drug holidays, but weight deficits may be related to these factors. Side effects did not correlate with dosage. The temporary growth deficits of the first year are of such minor magnitude as to have little clinical significance.