A 38-year-old woman with long-standing Wegener's granulomatosis had remission with cyclophosphamide therapy after failure of response to other agents. Severe cystitis with massive blood loss necessitated a cystectomy. The bladder contained an unsuspected neoplasm, histologically resembling leiomyosarcoma, but desmosomes seen on electron-microscopic examination suggested that this was a spindle cell carcinoma. The findings document occurrence of cyclophosphamide related bladder cancer in a patient without a previously diagnosed neoplasm and indicate a need for cytologic monitoring of the bladder in patients requiring therapy with this drug.