Prostaglandins of the E series have been shown to have immunosuppressive properties. To study the effects of the prostaglandin E1 analogue misoprostol on renal function and graft rejection after transplantation, we conducted a randomized, double-blind, placebo-controlled trial in 77 renal-allograft recipients. The subjects received misoprostol (200 micrograms four times daily by mouth; n = 38) or placebo (n = 39) for the first 12 weeks after transplantation, in addition to standard immunosuppression with cyclosporine and prednisone. They were then observed for an additional four weeks after the drug or placebo was discontinued. Treatment with misoprostol was associated with a significant improvement in renal function as judged by the mean (+/- SEM) serum creatinine concentration (128 +/- 7 vs. 158 +/- 11 mumol per liter after 12 weeks; P = 0.03) and creatinine clearance (84 +/- 6 vs. 69 +/- 5 ml per minute per 1.73 m2 of body-surface area; P = 0.05). There was a significant reduction in the incidence of acute rejection in the group treated with misoprostol as compared with the placebo group (10 of 38 vs. 20 of 39; P = 0.02), and there was less need for rehospitalization after transplantation (4 +/- 1 days with misoprostol vs. 10 +/- 2 days for placebo; P = 0.03). Although blood levels of cyclosporine did not differ significantly between the groups, they tended to be higher in the misoprostol group, as did the incidence of acute nephrotoxicity due to cyclosporine (13 of 38 vs. 8 of 39). Infectious complications tended to be fewer in the misoprostol-treated group (14 of 38 vs. 21 of 39). We conclude that misoprostol improves renal function and safely reduces the incidence of acute rejection in renal-transplant recipients treated concurrently with cyclosporine and prednisone.