Abstract Four patients heterozygous for familial hypercholesterolaemia were treated by repeated plasma exchange with or without lipid-lowering drugs. Repeated plasma exchange without drug therapy in 3 patients was associated with a significant 18–28% decrement in plasma cholesterol level, comparing control with plateau values observed 3 weeks after exchange. Further decrements in plateau values followed the addition of lipid-lowering drugs used in combination, clofibrate-nicotinic acid or clofibrate-nicotinic acid-cholestyramine (range of total decrement 39–50%). Plasma exchange was associated with an increased excretion of endogenous faecal steroids, but this increase was Completely abolished by the subsequent administration of clofibrate-nicotinic acid. This therapy prevented any increase in bile acid excretion with concomitant use of cholestyramine resin. Plasma exchange with drug therapy was associated with a sustained rise in plasma cholesterol specific radioactivity. In a fourth patient, clofibrate-nicotinic acid was administered prior to plasma exchange and led to a 24% fall in plasma cholesterol. Subsequent plasma exchange in this patient produced no sustained change in plasma cholesterol plateau level. In two patients, withdrawal of drugs allowed plasma cholesterol to return to pre-exchange control levels. These observations suggest that plasma exchange probably produced an increase in endogenous cholesterol synthesis and a mobilisation of tissue cholesterol. In relation to plateau cholesterol values 3 weeks after an exchange, the data suggested that the reduction in plasma cholesterol level with plasma exchange and drug therapy could have been achieved by intensive drug therapy alone.