BackgroundFollicular dendritic cell sarcoma is a very rare bladder tumor with very few cases that have been reported in the English literature.Case presentationWe report an unusual case of follicular dendritic cell sarcoma that is coexistent with urothelial carcinoma (UC) in the urinary bladder of a 73-year-old man, who first presented with lower abdominal pain. Microscopic examination of the first transurethral resection of bladder tumor (TURBT) sample showed a neoplasm containing spindle or ovoid-shaped cells that were arranged in storiform, nested or swirling patterns. Abundant mitotic Figs. (30 mitoses/10 high-power fields) and apoptotic bodies were present. The tumor cells were positive for CD21 and vimentin, partly positive for CD23, D2–40 and CD35. After 6 weeks, the tumor recurred lately, which surprisingly contained a component of urothelial carcinoma. The first TURBT sample was then reviewed and a coexisting UC mixed with FDCS was identified by examining the deeper levels of the tumor blocks.ConclusionsThis case is, to our knowledge, the first time to report the coexistence of FDCS and UC in the urinary bladder of an elderly patient. And these two tumors may share a similar molecular mechanism.