Inverted papillomas of the urinary bladder.

Study of 20 patients with inverted papilloma of the urinary bladder and urethra revealed that the lesion is a true neoplasm, benign in its histologic morphology and clinical behavior. The lesion is believed to arise from the trigone and bladder outlet as a result of chronic proliferative cystitis. It occurs predominantly in men who are past middle age. The most commonly associated clinical symptoms are hematuria and those of urinary obstruction. The lesion may be easily mistaken for a low-grade papillary transitional cell carcinoma, although the histologic appearance is distinctly different, as is its subsequent behavior.