Recently the World Health Organization published a new classification of urinary bladder tumors which is intended to take into account better the biology of the various lesions and to better distinguish between clearly benign and malignant lesions. We examine the possible diagnostic and clinical impact of the new classification, including recent immunohistochemical findings. Papillary urothelial lesions include papillomas, papillary neoplasms of low malignant potential, and papillary carcinomas. Flat urothelial lesions include hyperplasia, reactive atypia/atypia of unknown significance, dysplasia, and carcinoma in situ. Invasive patterns of papillary carcinomas are discussed, with special emphasis on lamina muscularis mucosae substaging. The most important feature of the new classification is its differentiation of two types of low-grade, noninvasive papillary urothelial lesions: papillary neoplasm of low malignant potential vs. papillary carcinoma. Long-term follow-up studies are needed to determine the clinical significance of this differentiation.
