Hepatic lipase (HL) is an important enzyme in the metabolism of triglyceride-rich lipoproteins and high density lipoproteins. The clinical syndrome of HL deficiency is rare and difficult to identify. We studied carriers of mutant HL to ascertain whether there are distinctive clinical and/or biochemical characteristics of the heterozygous state. In an Ontario kindred, compound heterozygosity for two HL mutations, S267F and T383M, underlies the clinical syndrome of complete HL deficiency. We report that simple heterozygotes for either HL mutant do not have a discrete lipoprotein abnormality, except for relative triglyceride enrichment of lipoprotein fractions with d > 1.006 g/mL. Postheparin HL activity is depressed to a greater degree in carriers of S267F compared with carriers of T383M. Retinyl palmitate loading studies in a compound heterozygote revealed impaired clearance of chylomicron remnants. The dyslipoproteinemia in a compound heterozygote was ameliorated by lovastatin. There was no difference in the quantity and distribution of HL mRNA in the liver of a compound heterozygote when compared with that of a normal subject. Thus, HL deficiency associated with structural variation of the HL gene is characterized by premature atherosclerosis, triglyceride enrichment of lipoprotein fractions with d > 1.006 g/mL, the presence of circulating beta-very low density lipoproteins, and abnormal catabolism of postprandial triglyceride-rich lipoproteins.