Risk of second primary malignancy was assessed in a population-based survey of persons who developed cancers of the renal parenchyma, renal pelvis, or ureter in Connecticut during the period 1935-1982. Among 4176 patients with a first primary tumor of the renal parenchyma, a second cancer was reported in 219 (5%), yielding a small but significantly elevated relative risk (RR) of 1.2, which reflects excesses for cancers of the bladder, kidney, and lymphatic-hematopoietic system. Among 939 patients with a first primary tumor of the renal pelvis or ureter, a second cancer was reported in 155 (17%), associated with a significantly elevated RR of 2.7. This resulted mainly from a 21-fold increase in risk for bladder cancer, although significant excesses were also found for lung and prostate cancers, and metachronous cancers of the renal pelvis and ureter. These associations seem to reflect the multicentric behavior of tumors arising in the urinary tract, the role of cigarette smoking, and host factors yet to be defined, and some degree of heightened medical surveillance and detection of tumors, especially in the same organ system.