The risk of cancer was analysed in a cohort of 604 male workers who had been engaged in manufacturing and/or handling benzidine and/or beta-naphthylamine during the period 1945-71 at two factories located in the city of Osaka. The cohort was followed up from 1 January 1970 to the date of death or 31 December 1986. The mean follow-up time was 16.1 years. A total of 84 deaths was found compared with 112.66 expected based on the mortality of the Osaka population. Thirty-six were found to be dead of malignant neoplasms; 9 stomach, 2 colon, 1 rectum, 3 liver, 1 bile duct, 1 pancreas, 1 maxillary sinus, 6 lung, 8 bladder, and 2 ureter neoplasms as well as 1 case of myeloid leukemia. Seven cases were ascertained on death certificates as neoplasms of uncertain behaviour, all of which were tumors of genitourinary organs except for one case of brain tumor. Cancers of genitourinary organs and tumors of uncertain behaviour showed statistically significant increased standardized ratios (SMR = 12.20, 4.89). The mean age of death of those having genitourinary organs including cancer was 59 years old, and the latent period between the first exposure and the occurrence of the disease was 19.7 years on average. Non-significant increased risks of cancers of the colon, rectum, liver and lung were observed among the workers exposed to benzidine. Among the 7 histologically confirmed cases of these cancers, there were 2 adenocarcinomas of the lung, 1 adenocarcinoma of the rectum, 2 hepatocellular carcinoma and 1 adenocarcinoma of the bile duct. Seven patients with genitourinary tumors were found to have died of other primary cancers.