Aim: Our aim was to compare the interobserver variability between the 1998 WHO/ISUP and 1973 WHO classifications. Methods: 258 consecutive papillary urothelial carcinomas were reviewed by two pathologists and assigned a tumor grade according to the 1973 WHO and 1998 WHO/ISUP without the knowledge of primary diagnosis and clinical follow-up. All cases were also histologically staged by the two pathologists separately as follows: pTa ( noninvasive), pT1 ( lamina propria invasion only), pT2 ( muscularis propria invasion). Findings of both pathologists and degree of agreement were compared statistically by using Pearson's chi(2) test and kappa statistics respectively. A kappa value of 0.21-0.40 is accepted as fair, 0.41-0.60 moderate and 0.61-0.80 substantial agreement. Results: Regardless of the pathologist, tumor grades of two classifications correlated to each other and the pathological stage ( p < 0.05). Overall degree of agreement between pathologists was higher in the 1998 WHO/ISUP ( kappa 0.59) than the 1973 WHO ( kappa 0.41), but both were still moderate. Papillary urothelial neoplasia with low malignant potential was the group of 1998 WHO/ISUP that showed the lowest degree of agreement and if excluded, interobserver variability of the 1998 WHO/ISUP decreased significantly ( kappa 0.84). Conclusion: The diagnosis of papillary urothelial neoplasia with low malignant potential and the criteria that differentiates it from low-grade carcinomas needs improvement in order to compare the different studies and therapies and to provide more accurate information for management. Copyright (c) 2007 S. Karger AG, Basel.