Between 7973 and 7977 we encountered 22 cases of acute renal failure after transplantation in 70 patients who received perfused cadaver kidneys. Nearly two-thirds of 76 nonfunctioning grafts were lost due to subsequent superimposed rejection, often undetected and, hence, untreated. Thirty-one percent of the 76 recovered function. The recovery rate, we believe, can be improved by earlier diagnosis and treatment of rejection and by avoiding invasive diagnostic procedures in the early postoperative period. If the oliguric period extends beyond two weeks, a closed percutaneous renal biopsy is justified. The diagnosis of rejection and/or other abnormality as well as subsequent treatment are very important in these patients