A 76-year-old man was found to have a false hypertriglyceridemia due to a 40-fold increased glycerolemia. This metabolic change was due to a deficiency in glycerol kinase (ATP:glycerol phosphotransferase, EC 2.7.1.30) activity in the cells of this patient as shown by incubation of his white blood cells with [14 C]glycerol. Several chromatographic analyses and quantitative assays were performed on plasma and urine of this patient and of his relatives. The small number of this family's members did not allow to specify the mode of transmission of this genetic trait.