A study of 35 cases of inverted papilloma of urothelium confirms that it is a distinctive entity and typically benign. These lesions occur mainly in males, may be multiple, and are sometimes situated elsewhere than in or near the trigone, which is their characteristic site. Their precise histogenesis remains uncertain. It must be stressed that not all transitional cell tumours of inverted pattern are of this type; some will have the appearance of and behave as malignant tumours. The same diagnostic criteria of malignancy established for the more common exophytic papillary neoplasms must be applied also to the inverted papillary tumours herein described.

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