A 53-year-old patient with primary HDL-deficiency is reported. About 2% of the normal concentration of alpha1 HDL was present in his plasma. The alpha1-high-density-lipoproteins separated into two fast-moving components in polyacrylamide gel electrophoresis. The Apo HDL contained both the main apolipoproteins, Apo A-I and Apo A-II, but in disproportionally reduced amounts, the concentration of Apo A-I being reduced about 360-fold, and that of Apo A-II about 14-fold. Concomitantly, the amount of the Apo C polypeptides in the HDL-fractions was decreased to about 5.5% and the activity of the enzyme lecithin cholesterol acyltransferase (EC 2.3.1.4.3) in plasma was found to be only 40% of normal. Apoprotein D was present in the LDL in association with Apo B, forming an abnormal, fast-moving LDL-complex. Apo A-I and Apo A-II were both of normal size as determined by SDS-PAGE, and reduction with thiols resulted in the shift of the M.W. of Apo A-II from 17,000 daltons to about 8,500 daltons. Both proteins were found in the same position as their normal counterparts in analytical isoelectric focusing. The most likely explanation for the multiple lipoprotein abnormalities seems to be that a defect in the regulation or structure of Apo A-I is the basis of the HDL-deficeincy.