OBJECTIVES: To determine whether loss of the tumour suppressor gene retinoblastoma (Rb) and increased expression of the p53 protein were associated with increased tumour cell growth fraction. PATIENTS AND METHODS: Tumours from 105 patients (72 men, 33 women; median age 69 years, range 35-89) with newly diagnosed primary transitional cell bladder carcinoma were studied. Tumour samples were taken by means of cystoscopic resection. Expression of the retinoblastoma (Rb) and p53 gene products was assessed immunohistochemically in 98 of the carcinomas. The proportion of cells expressing the Ki67 antigen (Ki67 index which is a measure of growth fraction) was determined in 64 cases. RESULTS: p53 protein was detectable in 50% and Rb protein in 82% of the tumours. Staining for p53 and lack of staining for Rb protein were associated with muscle-invasive growth and high tumour grade (G3). The Ki67 index varied over a wide range (1-47%), but there were significant differences between mean indices for poorly differentiated (G3) and well or moderately differentiated (G1/G2) tumours, and between indices for muscle-invasive and the remaining tumours. The mean Ki67 indices for Rb-negative tumours and p53-positive tumours were approximately twice those for Rb-positive and p53-negative tumours. Only 10% of the tumours expressed high levels of p53 protein and failed to express Rb. CONCLUSION: These observations are consistent with the hypothesis that loss of Rb and mutation and overexpression of p53 are associated with an increased tumour cell growth fraction and that such changes may play a role in the de-regulation of cell proliferation in transitional cell carcinoma of the bladder.