A biopsy of normal-looking mucosa showed mucosal abnormalities in 48.9% of patients with transitional cell carcinoma of the bladder. The presence of dysplasia or carcinoma in situ was significantly associated with grade but not with tumour category. In 100 patients who completed a course of radical megavoltage X-ray therapy, the result of mucosal biopsy gave no significant prognostic information about tumour regression, durable local control, survival or the risk of subsequent metastases.