In 1999, the World Health Organization (WHO) published a new classification of papillary urothelial tumors of the urinary bladder. Intended to represent a reproducible, easy-to-use classification system that better separates patients with true malignancies (bladder cancer) from those patients who are at an increased risk for developing bladder cancer, problems in the differential diagnosis of various lesions remained. Probably the most critical distinction is between papillomas, papillary urothelial neoplasms of low malignant potential (lmp), and grade I papillary carcinomas. Conversely, problems in the distinction between reactive atypia, atypia of unknown significance, and dysplasia, as well as the distinction of dysplasia from carcinoma in situ (CIS), are unresolved. Whether urothelial basal cell status assessment on hematoxylin and eosin-stained slides completed by cytokeratin immunohistochemistry with anticytokeratin clone 34betaE12 may help to improve some of the previously mentioned diagnostic dilemmas was investigated. Basal cell status assessment was helpful in the differentiation between dysplasia and CIS. In dysplasia, CK IHC showed a predominantly basal labeling pattern, whereas in CIS, labeling of all urothelial layers was seen. Basal cell status assessment could separate 2 groups of pTa GIb papillary carcinoma. Group 1 with a continuous basal CK labeling and a low MIB-1 labeling index (LI) was compared with group 2, with a diffuse labeling pattern and a significantly higher MIB-1 LI. Whether group 1 carcinomas should better be assigned to the group of papillary urothelial neoplasms of lmp is discussed.