A three-year-old asymptomatic boy with hyperlysinemia is presented. The patient's plasma lysine levels have been constantly high (685-1370 mumol/l) and excessive urinary excretion of ornithine, arginine and cystine have been noted. There was no detectable activity of lysine-ketoglutarate reductase nor saccharopine dehydrogenase in skin fibroblast culture. Review of the reported cases and this patient with serious biochemical defect but without symptoms indicate clinical heterogeneity in hereditary hyperlysinemia.