The fraction of proliferating cells in 178 transitional cell carcinomas (TCCs) was determined by proliferating cell nuclear antigen (PCNA/cyclin) immunostaining. The fraction of positive nuclei ranged between 0% and 100%. In WHO grade 1 tumours only occasional cells were positive for PCNA/cyclin, whereas nearly all of the nuclei in WHO grade 3 tumours were positive for PCNA/cyclin (p less than 0.0001). Nodular tumours showed a higher growth fraction than papillary tumours (p less than 0.0001). Superficial Ta-T1 tumours showed a significantly lower fraction on nuclei positive for PCNA/cyclin than muscle invasive tumours (p less than 0.0001). Tumours with pelvic lymph node metastasis also had a high growth fraction (p less than 0.0001). The growth fraction as determined by PCNA/cyclin immunostaining was significantly related to aneuploidy (p less than 0.0001, S phase fraction (p less than 0.0001) and mitotic frequency (p less than 0.0001). The results show that cell proliferation in the context of histopathology can be assessed by PCNA/cyclin immunostaining in TCC. The results suggest that PCNA/cyclin immunostaining has prognostic value in TCC and consequently is clearly a subject for further studies as a prognostic variable.