Although patients with superficial bladder cancer (Ta, T1) have a generally good prognosis, those patients who develop muscle-invasive tumours or metastatic disease at recurrence do poorly clinically. In the current study 69 patients undergoing complete transurethral resection for superficial transitional cell cancer of the bladder were investigated for different clinical and biological characteristics as possible prognostic factors: age, sex, performance of instillation therapy and immunohistochemical determination of mutational inactivation of p53 tumour-suppressor gene (monoclonal antibody PAb 1801) as well as immunohistochemical determination of the proliferation rate by staining for PCNA (proliferating cell nuclear antigen) (monoclonal antibody PC 10). After a median follow-up of 45.8 months, 12 of 14 patients (85.7%) with more than 20% of cells positive for p53 had disease progression with muscle-invasive growth compared with only one of 55 patients (1.8%) negative for p53 (P < 0.01, chi 2 test). During univariate analysis histological grade (G1 vs G2) (P = 0.0373), positivity for PCNA (> 60% of cells) (P = 0.0033) and positivity for p53 (P < 0.001) were significant prognostic factors for disease progression (log-rank test), while during multivariate analysis only positivity for p53 was a significant predictor for relapse of bladder cancer (P = 0.0029) (multivariate Cox regression analysis). The immunohistochemical detection of mutations of the p53 gene has been demonstrated to be a reliable, easily performed and thereby widely available technique for the investigation of fresh-frozen or paraffin-embedded tumour specimens. The results demonstrate the important role of the p53 tumour-suppressor gene protein in the development and for the progression of bladder cancer. If the high prognostic value of p53 mutations in superficial bladder cancer is confirmed in larger prospective trials, more aggressive therapeutic strategies could be discussed for patients with p53 mutations in their tumour specimens.