SummaryTwo enzymes, lipoprotein lipase and hepatic triglyceride lipase, are involved in the hydrolysis of triglycerides from chylomicrons and very low density lipoprotein (VLDL). Lipoprotein lipase has an absolute requirement for apolipoprotein CII for activity. Three inborn errors of metabolism which give rise to hypertriglyceridaemia have been described. The biochemical and clinical aspects of these disorders, lipoprotein lipase deficiency (familial type I hyperlipoproteinaemia), hepatic triglyceride lipase deficiency and apo‐CII deficiency are discussed.