The 1973 WHO (World Health Organization) classification of bladder tumours anticipated a probable need for eventual revision of the criteria for diagnosing papillary and flat bladder neoplasia. This is the revised classification, and a major new category has been introduced: papillary urothelial neoplasm of low malignant potential. Many of the tumours previously designated as papillary transitional cell carcinoma, grade I, now fall into that category. Another major change is in the designation of flat lesions, including a definition of carcinoma in situ. Finally, a number of variant forms of urothelial carcinoma are presented, along with new entities not recognized when the first edition was published.