Ureteral injuries occurring in 22 patients during surgical and irradiation therapy of abdominal cancer were studied, with the intention of improving the management of this operative hazard. The most frequent type of damage was physical trauma from accidents of surgical technique: puncture, crushing, or ligation of the ureter. Fibrosis, secondary to radiation of tumors close to the ureters, occurred in three patients. A majority of the injuries were diagnosed and repaired at the time they occurred; this is the best plan of management. When an injury was not noticed, the signs and symptoms usually observed in the immediate postoperative period were pain, fever, azotemia, anuria, and urine draining from an abnormal site. A blood-free operative field and careful surgical dissection will keep accidents at a minimum, but if one does happen the surgeon must be prepared to repair the ureter, with varied techniques, depending on the site, type, and extent of the injury. Preoperatively an outline of the urinary tract should be obtained (by roentgenography) and indwelling ureteral catheters inserted if possible.