The clinical correlates of methylphenidate blood levels in hyper-active children and normal adults were examined in five studies. Although occasional correlations between blood levels and neuroendocrine response were noted within subjects along the pharmacokinetic time profile of the drug, no significant associations were found between blood levels and clinical response in behavioral measures or laboratory tests of attention or activity. It is unlikely that routine methylphenidate blood level determinations will become a part of the routine clinical management of hyperactive children.