Inflammatory myofibroblastic tumor (IMT) of the bladder is an uncommon myofibroblastic spindle cell proliferation. Because of its cytologic features and infiltrative nature, it may be difficult to distinguish histologically from sarcomatous proliferations such as sarcomatoid urothelial carcinoma, leiomyosarcoma, and embryonal rhabdomyosarcoma. Recently, anaplastic lymphoma kinase (ALK) gene translocations or ALK protein expression in IMT has been reported, especially in patients of relatively young ages. However, there are only a few reports mentioning IMT of the bladder. We sought to determine the frequency of ALK expression among IMTs of the urinary bladder. We examined 16 cases of IMTof the bladder in 14 patients to elucidate the incidence of ALK-1 expression by immunohistochemistry and its diagnostic usefulness. The age of patients with IMT ranged from 18 to 76 years, with an average age of 42.8 years. The tumors from 10 of 14 patients (12 of 16 cases) were positive for ALK-1. ALK-1-positive cases ranged in age from 18 to 73 years (mean, 39.2 years; median, 38 years) and ALK-1-negative cases from 41 to 76 years (mean, 41.5 years; median, 44.5 years). Two locally recurrent cases were positive for ALK-1 in both the primary and recurrent lesion. ALK-1 immunostaining was detected only in the cytoplasm, with granular or subplasmalemmal linear features, suggesting ALK gene translocation. ALK-1 immunostaining was also performed in 8 sarcomatoid urothelial carcinomas, 5 genitourinary leiomyosarcomas, and 2 stromal tumors of uncertain malignant potential of the prostate, all of which were negative. These results support that ALK-1 immunostaining is useful to differentiate IMT from other malignant spindle cell neoplasms of the bladder. There were no histologic differences between ALK-1 positive and negative IMTs.