Hyperlipoproteinemia type V, with serum triglyceride concentrations of about 20 mmol/l, was detected in a pair of monozygotic, 40-year-old twin brothers. One of them had had recurrent attacks of pancreatitis, the other not. The endocrine and exocrine pancreatic functions were apparently normal, supporting that the pancreatitis was secondary to the hypertriglyceridemia. After successful lipid-lowering therapy the attacks of abdominal pain disappeared and remained absent during a 13-year follow-up period. The other twin died of ischemic heart disease nine years after the discovery of his lipid abnormality. The reduction of his lipid levels had been much less successful. His average "atherogenic index" (the ratio of cholesterol in low density to that in high density lipoproteins) was normal but increased to a very high value if cholesterol in very low density lipoproteins was also included together with the low density ones in the numerator. Lipoprotein particles modified in composition may have contributed to an increased uptake through a scavenger pathway and promoted atherosclerosis.