A prospective study of changes in plasma lipids after renal transplantation was performed in order to compare the effects of cyclosporin and conventional immunosuppression. Twenty-eight patients were studied, 18 of whom were allocated randomly to immunosuppression with either cyclosporin alone (nine subjects) or azathioprine and prednisolone (nine subjects). A further ten patients received cyclosporin and prednisolone. Total cholesterol, triglycerides and HDL, LDL and VLDL cholesterol subfractions were measured before transplantation, 21 and 90 days after transplantation, and also, in 12 patients (six on cyclosporin and prednisolone, and six on azathioprine and prednisolone), 2 years after transplantation. Triglycerides were initially elevated, and decreased after transplantation in all three groups. Total cholesterol was unchanged in the azathioprine and prednisolone group, whereas it increased significantly by 90 days in both the cyclosporin group and the cyclosporin and prednisolone group. This was due primarily to LDL cholesterol, which increased by 45% in the cyclosporin group and 28% in the cyclosporin and prednisolone group. Both total and LDL cholesterol remained elevated 2 years after transplantation in patients receiving cyclosporin and prednisolone, but were unchanged in the azathioprine and prednisolone group. There was no relationship between renal function and plasma lipid changes.