Objectives: To compare WHO 1973, WHO/ISUP 1998 and WHO 1999 histologic grading systems, and also to evaluate the primary (most common) and secondary (second most common) patterns of cancer growth according to these three grading systems. Material and Methods: The study consisted of 87 bladder transurethral resections that were classified as grade 1, 2 and 3, and papillary urothelial neoplasm of low malignant potential (PUNLMP), low grade (LG) and high grade (HG) carcinoma considering WHO 1973 and WHO/ISUP, respectively. The WHO 1999 system was subdivided high grade into grades 2 and 3 (HG-2 and HG-3). For combined scoring, primary (most common) and secondary (second most common) grades according to extension were recorded for three grading systems. The number was repeated when only grade was seen in all extension of the tissue examined. A final combined score was obtained which ranged from 2 to 6 for the WHO 1973 and WHO/ISUP 1998 systems and from 2 to 8 for the WHO 1999 schema. The TNM system was used for the pathologic staging. Results: When considering the pathological stage, there were statistical differences between the WHO 1973 grades (p=0.011 and p=0.000), and LG and HG carcinomas of WHO/ISUP 1998 (p=0.000) and also the WHO 1999 grades (p=0.010 and p=0.003), except PUNLMP. Regarding the combined scoring, significant differences were found between score 4 (2+2) and 5 (2+3) of WHO 1973 (p=0.014) and score 5 (LG+HG) and 6 (HG+HG) of WHO/ISUP 1998 (p=0.011). There was also a significant difference between scores 4 and 6, and 6 and 8 of the WHO 1999 combined scoring system (p=0.019 and p=0.019). WHO 1973, WHO/ISUP 1998 and WHO 1999 systems were positively correlated with the pathological stage (r(s)=0.30, r(s)=0.52 and r(s)=0.50, respectively), whereas there was weak association between the combined scoring systems and stage (r(s)=0.20, r(s)=0.18 and r(s)=0.19). Comparing these grading systems, the grade 2 of WHO 1973 was subdivided into LG and HG in WHO/ISUP 1998 and also LG-1 and HG-2 in WHO 1999 systems. The group of HG carcinoma in WHO/ISUP 1998 which was subdivided into HG-2 and HG-3 in the WHO 1999 system was different statistically in relation to the stage. Conclusions: Our results revealed that the WHO 1999 system may be more useful to evaluate the bladder carcinoma histopathologically in comparison to the WHO 1973 and WHO/ISUP 1998 systems. Copyright (C) 2004 S. Karger AG, Basel.