The value of preoperative intravenous urography (IVU) to prevent iatrogenic damage to the urogenital tract was investigated in a series of 236 females and 188 males (mean age 54 [5-87] years) who underwent an operation on the colon or rectum between April 1988 and March 1992. The operations were: 101 right and 49 left hemicolectomies, 125 sigmoid resections, 74 anterior rectum resections, 58 abdomino-perineal rectum amputations and 17 total colectomies. Preoperative IVU was performed in 279 patients (65.8%), while in 145 (34.2%) urgency of the operation or intolerance to contrast medium prevented the procedure being done. The results were abnormal in 75 of the 279 IVUs (26.9%). Of the latter, only 19 (6.8%) of the abnormalities were related to the colorectal disease (renal obstruction due to tumour: 9, malignant invasion of the bladder: 5, ureter displacement: 5). Other diagnostic procedures had given abnormal results in six patients, although the preoperative IVU had been unremarkable. Iatrogenic damage to a ureter occurred in only one patient: the IVU had been normal. No ureteral damage occurred in any of the patients who had not had an IVU. These data indicate that the decisive factor in preventing intraoperative damage to a ureter is not a preoperative IVU but careful intraoperative dissection and visualization of the ureter. There is, therefore, no need for routine preoperative IVU in these cases.