Hyperglycerolaemia and glyceroluria is described in a patient who presented transient dietary-induced hypertriglyceridaemia. The increased glycerol in blood and urine was related to glycerokinase deficiency as evidenced by a hepatic tissue glycerokinase activity of one tenth (4.8 X 10(-5) mmol g-1 min-1) of that of normoglycerolaemic subjects (5.9 X 10(-4) ). As triglycerides are routinely determined by an enzymatic method after hydrolysis to glycerol, the differential diagnosis between true hypertriglyceridaemia and hyperglycerolaemia may prove difficult. Persisting "hypertriglyceridaemia' despite adequate dietary and drug treatment should suggest the existence of undiagnosed hyperglycerolaemia .