AbstractAIB1, a candidate oncogene in human breast cancer, is frequently amplified and overexpressed in several types of human cancers, but the status of AIB1 amplification and expression in urothelial carcinoma of the bladder (UC) and its clinical/prognostic significance is unclear. In our study, the methods of immunohistochemistry and fluorescence in situ hybridization were utilized to examine protein expression and amplification of AIB1 in 163 primary UCs and in 30 samples of normal bladder mucosa. Overexpression of AIB1 and amplification of AIB1 was found in 32.5 and 7.0% of UCs, respectively. In univariate survival analysis of the UC cohorts, a highly significant association of overexpression of AIB1 with shortened patient survival (mean: 45.6 months vs. 59.0 months, p < 0.001, log rank test) was demonstrated. In different subsets of UC patients, overexpression of AIB1 was also a prognostic indicator in grade 1 (p = 0.007), grade 2 (p = 0.010) and grade 3 (p = 0.015) tumor patients, and in pTa (p = 0.025), pT2–4 (p = 0.004), pN0 (p < 0.001) and pT2–4/pN0 (p = 0.040) tumor patients. Importantly, AIB1 expression (p < 0.001) together with pT and pN status (p < 0.05) provided significant independent prognostic parameters in multivariate analysis. In addition, a significant correlation (p < 0.05) of overexpression of AIB1 with an increased UC labeling index of Ki‐67 (a cell proliferation marker) was observed in these UCs. Thus, these findings provide evidence that an overexpression of AIB1, as detected by immunohistochemistry, is an independent molecular marker for poor prognosis (shortened survival time) of patients with UC. © 2008 Wiley‐Liss, Inc.