Spindle cell proliferations of diverse types which vary greatly in their behavior may occur in the urinary bladder. Some of them such as the inflammatory pseudotumor and the postoperative spindle cell nodule are reactive and clinically benign although they may be responsible for significant symptoms. On the other hand, certain other lesions such as sarcomatoid carcinomas are typically highly malignant tumours. The features of the inflammatory pseudotumor and postoperative spindle cell nodule have only recently been defined. The tendency of the former to occur in young patients and the association of the latter with a recent operative procedure are important pieces of clinical information which may prevent their mis-diagnosis. The diagnosis of sarcomatoid carcinoma should always be considered when a malignant spindle cell proliferation is encountered in the urinary bladder. Careful search for minor foci of obvious epithelial differentiation is important in establishing the diagnosis which may also be aided by immunohistochemical staining for epithelial markers. Sarcomatoid carcinoma should be distinguished from the rare transitional cell carcinoma with pseudosarcomatous stroma and from carcinosarcoma. The final lesions briefly reviewed here are mesenchymal tumors both benign and malignant, which generally do not pose the same degree of diagnostic difficulty as non-neoplastic mesenchymal proliferations and sarcomatoid carcinomas.