Plant sterols have been shown to reduce dietary cholesterol absorption and hence, total and low-density-lipoprotein (LDL)-cholesterol concentrations in humans. In this study the cholesterol-lowering effects of dietary supplementation with the hydrogenated plant sterol sitostanol (3 g/d) were tested in 33 men with moderate hypercholesterolemia who were consuming an outpatient diet in which dietary cholesterol was restricted to < 200 mg/d. Sitostanol therapy did not significantly lower LDL cholesterol compared with the diet alone. Similarly, sitostanol therapy in conjunction with a cholesterol-lowering regimen of diet and 8 g cholestyramine did not significantly lower LDL-cholesterol concentrations. Hence, although previous reports have suggested that low-dose sitostanol therapy is an effective means of reducing LDL-cholesterol concentrations, its effectiveness may be attenuated when the diet is low in cholesterol.