The effects of cyclosporin and prednisone on plasma lipid and lipoprotein levels were studied in 20 allogeneic bone-marrow transplantation patients receiving cyclosporin plus prednisone therapy, and in 14 allogeneic patients treated only with cyclosporin during 100 days. Eighteen autologous bone-marrow patients not requiring cyclosporin were used as a control group. Patients were studied 5 days prior to transplantation, and on days 30, 60, and 100 after transplantation. To determine the reversibility of the changes, lipid parameters were analyzed 30 days after completion of the treatment. Nutritional supplementation, conditioning regimens, and concomitant medications were not significantly different between groups. Furthermore, no significant differences in age, weight, lipid, or lipoprotein levels were found at baseline. Our results indicate that cyclosporin therapy induces a reversible increase of plasma cholesterol, LDL-cholesterol, triglycerides, VLDL-triglycerides, and apolipoprotein B and a decrease of HDL-cholesterol, HDL2-cholesterol, and apolipoprotein A-I. The addition of prednisone to cyclosporin therapy induces a higher increase in plasma cholesterol mainly due to an increase in HDL-cholesterol. Total cholesterol/HDL-cholesterol ratio increased significantly in patients treated only with cyclosporin. No differences were found in this ratio in patients treated with prednisone compared to those submitted to autologous bone-marrow transplantation. Lipid changes observed in this study were reversible 30 days after cessation of cyclosporin treatment.