We assessed the effect of the prostaglandin E1 analog misoprostol on cyclosporine nephrotoxicity in patients with rheumatoid arthritis (RA). Thirteen patients with RA were given cyclosporine with misoprostol tablets, 800 micrograms/day for one week in a randomized, double blind, placebo controlled crossover trial. All had cyclosporine nephrotoxicity, documented by an increase in serum creatinine of at least 15% over the values before the start of cyclosporine treatment. Mean glomerular filtration rate (GFR) (single shot 51Cr-EDTA plasma clearance) at baseline was 77.3 ml/min (SD, 22.0). After misoprostol, it was 80.0 ml/min (SD, 18.9); after placebo, 79.1 ml/min (SD, 20.0). None of these changes were statistically significant. Serum creatinine levels and whole blood cyclosporine levels were also unchanged. Power to detect at least a 5 ml/min rise in GFR was 0.92. Short term misoprostol treatment does not improve the GFR of patients with RA on cyclosporine.