A case of invasive bladder cancer accompanied with enlarged pelvic lymph nodes, which were interpreted as metastasis, is reported. The lymphadenopathy eventually regressed spontaneously while the bladder cancer increased in size. Total cystectomy and pelvic lymph node dissection were performed. Histopathological examination of the lymph nodes revealed dilatation of the sinuses and marked increase in the number of histiocytes, which suggested sinus histiocytosis. Sinus histiocytosis is often associated with malignant tumor. It is considered as a sign of the host's immune activation with favorable prognostic significance. Nevertheless, such lymphadenopathy mimics a metastatic lymph node, making proper staging difficult.