Monoclonal antibody CA-50 is a useful tumour marker in gastrointestinal carcinoma. We report an assessment of its value in bladder cancer patients. We found raised levels in 2 of 2 patients with infiltrating carcinoma and in 8 of 35 with superficial carcinoma. The recurrence rate was higher in patients with raised levels, since 6 of 8 with elevated levels and 15 of 27 with normal values had a recurrence within 6 months. T1 and T3 carcinoma had a mean CA-50 level higher than normal. G3 tumours had a mean level slightly above normal. A statistically significant difference emerged when comparing Ta with T1 + T3 carcinomas. A longer study, with serial determinations, could assess the role of CA-50 as a prognostic indicator in bladder cancer.