Cholestyramine and bezafibrate were compared individually and in combination in the treatment of 18 patients with heterozygous familial hypercholesterolaemia. The study used a double blind, placebo controlled block design with a placebo run in period of two months followed by three phases of active treatment, each of two months' duration. Patients were randomly allocated to one of the six possible sequences of medication so that three patients would be treated with each sequence. Two patients withdrew from the study before completion. The median concentration of total cholesterol decreased from 9.65 mmol/l (interquartile range 8.62 to 8.72) to 7.24 mmol/l (6.70 to 7.52) with cholestyramine, to 8.09 mmol/l (7.18 to 8.68) with bezafibrate, and to 6.31 mmol/l (5.84 to 7.27) with the combination. This fall was due almost entirely to a decrease in the low density lipoprotein cholesterol concentration, and the combination was significantly more effective than either drug alone. The 98% confidence intervals for the median differences between the combination and cholestyramine and the combination and bezafibrate were 0.04 to 1.49 mmol/l and 0.51 to 2.18 mmol/l respectively. These results suggest that this combination is an effective and useful treatment in heterozygous familial hypercholesterolaemia.