Erythrocyte sodium-lithium countertransport (SLC) was measured in 17 patients with either combined hyperlipidaemia or hypercholesterolaemia before and after lipid lowering therapy. Before treatment SLC related to the serum triglyceride level and was increased in combined hyperlipidaemia. After treatment the SLC had returned to normal and the change in SLC was related to the change in serum triglyceride levels. Raised SLC is associated with essential hypertension but is not related to blood pressure. Therefore, the association of raised SLC with hyperlipidaemia and essential hypertension appears to have different underlying mechanisms.