"Nephrogenic adenoma": an unusual metaplastic change of urothelium.

"Nephrogenic adenoma" is a papillary bladder lesion of uncertain pathogenesis. We have studied 2 patients with this lesion for several years with serial biopsies in both cases. We believe that the natural history of "nephrogenic adenoma" and its invariable association with bladder irritation, coupled with the histopathologic and ultrastructural findings, strongly suggest that this "tumor" represents a papillary and glandular transformation of urothelium, in response to prolonged exposure to various injurious agents. It is best regarded as a metaplastic process rather than a hamartoma or a benign neoplasm. There is no reasonable basis for supposing that the process originates from mesonephric-derived epithelium. While not itself neoplastic nephrogenic metaplasia (our preferred term) is rarely associated with the late development of invasive but not, to date, metastasizing neoplasia. Routine cystoscopy would seem the most appropriate means of following these patients.