Erythrocyte assays for hypoxanthine guanine phosphoribosyltransferase (HGPRT) activity performed on two male half-siblings with hyperuricemia, produced results consistent with classic Lesch-Nyhan syndrome. Due to the absence of neurologic abnormalities, cognitive deficits, or self-mutilation, HGPRT activity was measured by intact fibroblast assay which revealed partial enzyme deficiency. The presence of an unstable mutant enzyme may have led to the discrepancy between the erythrocyte and fibroblast studies. This discrepancy emphasizes the difficulty in assaying this enzyme solely utilizing red blood cell studies to determine a patient's course. In order to provide an accurate prognosis and relevant genetic counseling, measurement of this enzyme utilizing intact fibroblasts is critical after establishing a diagnosis of HGPRT deficiency in a hyperuricemic male lacking typical clinical manifestations of Lesch-Nyhan syndrome, but having enzyme activity of erythrocytes consistent with the diagnosis.