The CD40 receptor is expressed in many immune cell types and is known to play a central role in both humoral and T-cell-mediated immunity, being a subject of intense research interest in recent years. It is also expressed on a variety of carcinomas and may therefore be of biological significance in the development and treatment of cancer. The expression of CD40 was examined immunohistochemically in a series of 131 bladder transitional cell carcinomas and the correlation with known prognostic markers and clinical outcome assessed. Seventy-eight per cent of the tumours were CD40-positive, with a highly significant association with both lower stage and lower grade (p0.001). Ta and T1 tumours expressed CD40 in 89 per cent of specimens compared with 62 per cent seen in T2-T4 tumours and in contrast to normal urothelium, which was mainly CD40-negative. CD40 expression was not related to any other clinicopathological variable including Bcl-2 and p53 expression, nor was it an independent prognostic marker. The lack of the relationship with Bcl-2 staining which is normally seen in basal epidermal cells may indicate alternative or abnormal CD40-mediated cell differentiation mechanisms. The diffuse expression seen in Ta bladder tumours may account for its clinically less aggressive behaviour and is likely to be an important factor in the excellent clinical response seen to BCG immunotherapy. It also raises the possibility of the future development of CD40/CD40 ligand-based immunotherapy for bladder cancer.