Pax (paired box) genes comprise a gene family crucial for cell differentiation that encodes a set of transcription factors. Recently, Pax-5 mRNA expression was suggested as a prognostic marker in bladder cancer (BC). However, a functional role of Pax-5 in BC is questionable because the protein expression was not determined in these studies. Therefore, we evaluated Pax-5 protein expression in an unselected, consecutive series of BC.We immunohistochemically investigated Pax-5 protein expression in 100 archival bladder tumours and 22 normal urothelial samples using tissue microarray (TMA) technology and a monoclonal antibody against Pax-5. Staining intensity and percentage of positively stained cells were determined and correlated to histopathological characteristics of the tumours and clinical follow-up data.All 22 samples of histopathologically normal urothelium were negative for Pax-5 protein expression. Overall, 70 of 100 tumours gave interpretable results. Only seven of 70 (10%) cases showed a positive nuclear Pax-5 staining but without significant correlation to clinicopathological characteristics. Interestingly, we could observe Pax-5 positive lymphocytes located within the tumour or closely adjacent in the underlying stroma in 24 of 70 (34%) cases in our series.Pax-5 protein expression is infrequent in BC. Absence of correlation to clinicopathological characteristics suggests a minor functional role of Pax-5 in BC. Pax-5 positive lymphocytes within reactive infiltrates adjacent to the tumour warrant further studies evaluating biological, immunological and clinical relevance.