A study of the relatives of a patient with apolipoprotein C-II deficiency showed that the defect is inherited as an autosomal recessive trait. The kindred studied originated from an isolated population in which considerable inbreeding has occurred for 140 years. Seven homozygotes had marked fasting chylomicronemia and triglyceridemia, and lacked detectable apolipoprotein C-II by several assay methods. Five homozygotes had experienced one to many attacks of pancreatitis from as early as six years of age. Obligate heterozygotes had apolipoprotein C-II concentrations about 30 to 50 per cent of normal values and had normal plasma triglyceride concentrations. This metabolic defect should be considered in patients with markedly elevated plasma triglycerides who have apparent lipoprotein lipase deficiency, and usually also have pancreatitis.