A 45-year-old woman with corneal opacification and severe coronary artery disease was noted to have the following plasma lipid levels (mg/dl, +/- SD): total cholesterol 111 +/- 13, triglyceride 62 +/- 6, very low density lipoprotein cholesterol 4 +/- 1, low density lipoprotein cholesterol 106 +/- 14, and high density lipoprotein (HDL) cholesterol 1 +/- 1 (normal, 50 +/- 14). Her two offspring and one brother were found to have HDL cholesterol values (mg/dl) of 23, 20, and 20, respectively. The percentage of cholesterol in the esterified form in the patient's plasma was normal at 70%. Lipoprotein electrophoresis showed no alpha lipoprotein band, and no HDL was detectable when plasma was subjected to analytic ultracentrifugation. Only trace amounts of lipids were noted within the HDL density region following preparative ultracentrifugation. Mean plasma apolipoprotein (apo) A-ll, apo B, and apo C-ll plasma levels were 13.8%, 130.6% and 26.6% of normal, respectively. The ratio of apo B to cholesterol within LDL was elevated. Apo A-l, the major HDL protein constituent, was immunologically undetectable in this patient's plasma. A decreased HDL cholesterol concentration has been associated with premature coronary artery disease. These data indicate that plasma apo A-l absence results in a striking reduction in HDL, is associated with premature coronary artery disease, and represents a new distinct disease entity.