A case of an unusually large pseudomalignant spindle-cell proliferation (or inflammatory pseudotumor) of the urinary bladder in a young man is presented. The only symptom was urinary frequency. The clinical and radiologic evaluation (ultrasonography, computed tomography, and magnetic resonance imaging) revealed a massive tumor (11 x 7.5 x 5.5 cm) arising from the bladder wall that was eventually resected without compromising bladder function. Although the histologic findings initially suggested a sarcoma, the immunohistochemical findings were consistent with that of an inflammatory pseudotumor. The patient remained asymptomatic, without evidence of recurrence, after 12 months of follow-up. Awareness of this inflammatory pseudotumor and its immunophenotypic characteristics is important to avoid an erroneous diagnosis and unnecessarily aggressive therapy.