The biopsies from 75 patients with transitional cell carcinoma of the bladder (25 Ta-T1; 45 T2-T4, 5M) were studied immunohistochemically for the expression of beta-human chorionic gonadotrophin (beta-HCG). Only 5 Ta-T1 tumours contained a small number of beta-HCG positive cells but 24 invasive tumours and all patients with metastases showed increased numbers of positive cells. A significant correlation was found between beta-HCG immunoreactivity and tumour category. In 30 patients with muscle-invasive disease (T2-T4,N0,M0) who were treated with radical radiotherapy a significant correlation was observed between response to treatment and beta-HCG expression; beta-HCG positive tumours did not respond to treatment. A difference in survival was found between patients with tumours negative for beta-HCG compared with patients with positive tumours, all treated with radical radiotherapy. The results indicate that beta-HCG expression increases with tumour invasiveness and the use of immunohistochemistry may prove a useful means of identifying radioresistant and aggressive forms of bladder cancer.