We studied two sisters 29 and 31 years old who had skin and tendon xanthomas, corneal clouding, and severe coronary atherosclerosis. Histologic examination showed collections of lipid-laden histiocytes in the skin. The patients' plasma cholesterol concentrations were 177 and 135 mg per deciliter (4.58 and 3.49 mmol per liter). Levels of high-density-lipoprotein cholesterol were 4 and 7 mg per deciliter (0.1 and 0.2 mmol per liter). Only traces of apolipoprotein A-I were detected in whole plasma. The plasma density fraction from 1.06 to 1.21 g per milliliter contained no high-density lipoprotein on high-pressure liquid chromatography, no apolipoprotein A-I on sodium dodecyl sulfate electrophoresis, and only traces of apolipoprotein A-I on radioimmunoassay. Apolipoprotein C-III was also not detectable. The activity of lecithin-cholesterol acyltransferase was 40 per cent of normal. The half-life of infused normal high-density lipoprotein was three days (normal, 5.8 days). The parents and children of these two patients had low levels of high-density-lipoprotein cholesterol and apolipoprotein A-I. These cases support the hypothesis that low concentrations of high-density lipoprotein promote atherosclerosis.