We report 5 cases of micropapillary urothelial carcinoma (MPUC) involving the renal pelvis (2), renal pelvis and ureter (2), and proximal ureter (1). The patients were 2 women and 3 men, ages 65 to 92 years (mean, 76.0 years). All tumors showed a high-grade transitional cell carcinoma component, and in 3 cases, there also were areas of in situ carcinoma. The case involving only the ureter occurred in a 65-year-old man with a history of nephrectomy 12 years previously for urothelial carcinoma of the renal pelvis. The tumor recurred in the ureteral stump. In all cases, areas displaying micropapillary architecture were observed. In 2 cases the micropapillary areas were noninvasive; in 1 case a pure invasive pattern was seen; and in 2 cases a mixed invasive and noninvasive pattern was present. the micropapillary pattern was invasive; and the case involving the ureteral stump contained invasive and noninvasive micropapillary carcinoma. All patients died of their tumors from 3 to 24 months after initial diagnosis. MPUC involving the renal pelvis and ureter is associated closely with advanced stages of disease and has highly aggressive behavior. Recognition of this growth pattern is important for prognosis and avoiding misdiagnosis with papillary renal cell carcinoma and other tumors.