Discordance between clinical phenotype and the level of a mutant enzyme activity may reflect differences between enzyme function in vivo and that measured by the customary enzyme assays on cell extracts. In the present study, the conversion of hypoxanthine to phosphorylated products was measured in intact skin fibroblasts and in cell extracts from seven patients with mutant hypoxanthine guanine phosphoribosyltransferase (HPRT) and six control subjects. The patient's phenotypes ranged from asymptomatic hyperuricemia to the Lesch Nyhan syndrome. Although there was a general correlation between the HPRT activity in cell extracts assayed by the usual methods and the function of the purine salvage pathway in patients, as reflected by urinary oxypurine excretion, there were notable exceptions. A more accurate appraisal of the functioning of the pathway at the cellular level is achieved by measuring the conversion of substrate to product in the intact cell at physiological concentrations of substrates, activators, and product and metabolite inhibitors, and in a physiological ionic environment. In one of the seven patients, the standard enzyme assay indicated normal function, whereas measurements in the intact cell exposed severe dysfunction of the salvage system. In another, the standard assay suggested a severe deficiency not evident in the intact cell or in the patient.