The effect of lovastatin on erythrocyte membrane composition and fluidity was investigated in eight patients with severe hypercholesterolaemia (mean LDL-cholesterol of 7.2 mmol l-1). Lovastatin was administered at a dosage of 40-80 mg for 20 weeks and was discontinued for 5 weeks thereafter. Parallel to a 47% fall in plasma LDL cholesterol, there was a significant reduction (P < 0.01) in erythrocyte membrane cholesterol:phospholipid molar ratio, while erythrocyte membrane fluidity assessed by diphenylhexatriene (DPH) fluorescence polarization increased significantly (P < 0.01). Discontinuation of lovastatin resulted in the reversal of erythrocyte membrane composition and fluidity to pre-treatment values.