High-grade papillary urothelial carcinomas display marked cytologic and architectural atypia. Within the papillary fronds, the neoplastic cells are arranged haphazardly with the loss of polarity. There is considerable variability in nuclear size and shape. The nuclei are hyperchromatic and prominent nucleoli are present. Mitotic activity is increased and mitoses may be found at all levels of urothelium. Some cases show numerous apoptotic cells.