The dose-response relationship of activated charcoal in reducing serum cholesterol was determined and the effects of charcoal and cholestyramine were compared in patients with hypercholesterolaemia. In a cross-over study 7 patients ingested charcoal 4, 8, 16 or 32 g/day, and finally bran, each phase lasting for 3 weeks. Serum total and LDL-cholesterol were decreased (maximum 29% and 41%, respectively) and the ratio of HDL/LDL-cholesterol was increased (maximum 121%) by charcoal in a dose dependent manner. Ten further patients with severe hypercholesterolaemia ingested daily for 3 weeks, in random order, activated charcoal 16 g, cholestyramine 16 g, activated charcoal 8 g + cholestyramine 8 g, or bran. The concentrations of total and LDL-cholesterol were reduced by charcoal (23% and 29%, respectively), cholestyramine (31% and 39%) and their combination (30% and 38%). The ratio of HDL/LDL-cholesterol was increased from 0.13 to 0.23 by charcoal, to 0.29 by cholestyramine, and to 0.25 by their combination. Serum triglycerides were increased by cholestyramine but not by charcoal. Other parameters, including the serum concentrations of vitamin A, E and 25(OH)D3 remained unaffected. The changes in lipids only partly subsided during the 3-week bran phase. In general, the acceptability by the patients and the efficacy of activated charcoal, cholestyramine and their combination were about equal, but there were individual preferences for particular treatments.