Despite the common anomaly of a duplicated collecting system in the urinary tract, urothelial cancer in a duplicated collecting system is a rare occurrence. Herein, we present 2 cases of renal pelvis tumor and 1 case of a ureter tumor which coexisted with a duplicated collecting system. One of the renal pelvis tumors developed bilateral transitional cell carcinoma within the bilateral duplicated pelvis. This has not been reported previously. The tumor of the ureter in the latter case was located at the junction site of the bifurcation. This finding is consistent with the postulation that urine reflux chronically irritates the distal segment of a duplicated ureter, rendering this segment susceptible to malignant change. Recurrence of the tumor is frequently observed, which necessitates an early diagnosis and radical treatment.