We have evaluated the effect of prostaglandin E1 (PGE1) on renal function after cardiac surgery with cardiopulmonary bypass in 20 patients, ten of whom received 0.02 mug . kg-1 . min-1 of PGE, by infusion into the oxygenator during bypass; ten patients served as controls. Serum beta2-microglobulin fell significantly and urine beta2-microglobulin increased significantly after surgery in both groups. Urine N-acetyl-beta-D-glucosaminidase was high after surgery in both groups, but it was significantly lower in the PGE1 group. Free water clearance fell significantly on the 1 st, 3 rd, and 5 th postoperative days compared with preoperative values in the control but not in the PGE1 group. These results suggest that PGE, may prevent renal dysfunction after cardiopulmonary bypass.