Isolated glycerol kinase deficiency (GKD) is an X linked recessive disorder. The clinical and biochemical picture may vary from a childhood metabolic crisis to asymptomatic adult "pseudohypertriglyceridaemia", the result of hyperglycerolaemia. We performed glycerol kinase (GK) gene analysis to study the molecular heterogeneity and genotype-phenotype correlation in eight males from three families with isolated GKD. All patients had hyperglycerolaemia and glyceroluria. Four patients from two families were essentially free of symptoms. Three patients had gastrointestinal symptoms with ketoacidosis or hypoglycaemia or both. One patient had recurrent convulsions as the only acute sign, without evidence that it was correlated with a catabolic state. Fasting tests in two symptomatic patients of family 1 showed hyperketotic states, together with a tendency to hypoglycaemia. The diagnosis was confirmed by a defective 14C-glycerol incorporation into trichloroacetic acid precipitable macromolecules in intact skin fibroblasts. Mutation screening of the GK gene was performed by amplification and direct sequencing of exons using PCR. Three novel mutations were identified: (1) a deletion starting downstream of exon 9, extending to the 3' end of the gene; (2) a nonsense mutation R413X caused by a C1351T transition; and (3) a missense mutation W503R caused by a T1651C transition. In addition, we found differences from the reported sequence: (1) exon 9 actually consists of two exons, which consequently will change the number of GK gene exons from 19 to 20 exons, and (2) nucleotide differences in exon 19. So far, no genotype-phenotype correlation can be established in these GKD families.