The risk of second primary cancer was evaluated in 29,128 patients who developed tumors of the urinary tract, including benign and malignant tumors of the renal pelvis and ureter and bladder papillomas in Denmark between 1943 and 1980. Among 9,162 persons with kidney cancer, 416 developed a second primary tumor [relative risk (RR) = 1.4]. Among 19,966 persons with bladder cancer, 1,423 developed a second primary tumor against 1,239 expected (RR = 1.1). The risk of bladder cancer was increased following kidney cancer in both men (RR = 6.3) and women (RR = 10.1), and kidney cancer was increased in both men (RR = 2.9) and women (RR = 4.5) following bladder cancer. These risks were particularly pronounced for cancers occurring in the ureter and renal pelvis. Etiologic similarities are likely explanations for these observations, which also emphasize the role of host factors and the multifocal nature of urothelial tumors. A decrease in relative risks since diagnosis of the first primary cancer was seen that may partly be attributed to a lessening of the intensity of medical surveillance with time. Among long-term survivors with kidney cancer, increased risks were observed for colon and pancreatic cancers, which may be related to treatment; approximately 25% received radiotherapy. Among bladder cancer patients, increased risks of cancers of the lung and larynx occurred, probably due to tobacco smoking. A slight elevation of prostate cancer (RR = 1.3) may be attributable to medical surveillance. Unexpected findings were the significant deficits of cancers of the stomach and rectum among patients with bladder cancer and stomach cancer among those with kidney cancer.