The outcome of patients with acute renal failure following cadaveric renal transplant has been evaluated in a prospective, controlled trial, comparing treatment with cyclosporine (CSA) to prednisone, azathioprine, and antilymphocyte globulin (AZA). There was a high incidence of acute post-transplant renal failure in both groups: 37 of 51 CSA and 31 of 45 AZA patients, due to the long exposure of kidneys to warm and cold ischemia. Onset of adequate renal function was delayed for three or more weeks in 27 (53%) CSA and only nine (20%) AZA patients, and the only predisposing factors found was donor hypotension. All nine AZA and 18 of the 27 CSA patients with prolonged oliguria subsequently had a spontaneous diuresis. Nine of the CSA patients were changed to azathioprine and prednisone because of suspected CSA toxicity, and eight of these kidneys began functioning within days, even though they had been oliguric for 21 to 83 days. Of these nine patients, five had adequate long-term function on AZA, three developed CMV infections that were fatal to two individuals, and two rejected their grafts. Plasma CSA levels fluctuated widely in all patients, but were not higher in any group, including those with prolonged oliguria. During the oliguric period, biopsy specimens proved rejection was more common in the nine patients who had their CSA stopped than in the other CSA patients, and seven of these nine developed a diffuse interstitial fibrosis that was thought to be a manifestation of CSA toxicity. Extensive analysis of the post-transplant course showed rejection and CSA nephrotoxicity as the only probable causes of the delayed recovery from oliguria in the CSA group. Although CSA-treated patients tended to have higher serum creatinines, no significant differences were found at 12 months in any of the groups. Overall graft survival was similar in all groups.