Lysosomal acid lipase activity was measured in mononuclear leukocytes of patients selected on the basis of premature cardiovascular disease, with or without hyperlipidemia. Enzyme activity was significantly lower in the patient population (4.8 +/- 1.3 nmol/min/mg protein, n = 190 males) than in an age-matched control population (5.4 +/- 1.3 nmol/min/mg protein, n = 124 males). There was no effect of hypercholesterolemia or hypertriglyceridemia on the enzyme activity. In the group of patients with normal plasma lipids (n = 77), 18% had mononuclear leukocyte acid lipase activity which fell below the control population 5th percentile, and in the range of enzyme activity observed in cells from obligate heterozygotes for inherited acid lipase deficiency (Wolman disease and cholesteryl ester storage disease). Studies of acid lipase activity in families of our patients provided evidence that an autosomal mutation is associated with (or responsible for) this reduced enzymatic activity and may represent an independent risk factor for the premature development of atherosclerosis.