The validity of urine cytology surveillance of occupational bladder tumor incidence was evaluated by reviewing the clinical history of nine bladder tumor cases found in dyestuff plant workers. A bladder tumor surveillance system has been organized for workers (179) active and 65 retired) who had been exposed to benzidine or beta-naphthylamine in the plant. The system consists of two-stage tests: periodic urine cytology as the first-stage screening test, followed by, in suspicious cases, urological examinations as second-stage tests. The urine cytology surveillance was found to be useful for two reasons: first, it detected tumors in five out of six cases surveyed by athe system, and second, four cases screened by cytology had tumors curable by transurethral operation, while other cases underwent cystectomy. Calculated average latent periods from the first and last exposure was 26.4 years (SD = 6.0) and 14.0 years (SD = 6.2), respectively. This time period indicates that the surveillance system should continue to be used for the exposed group even after workers have ceased being exposed.