Formaldehyde, a genotoxic and potent animal carcinogen, is widespread in the working environment as well as in private homes. The risk for cancer morbidity in Denmark during 1970-84 was estimated from standardized proportionate incidence ratios (SPIR) among men whose longest employment had been held since 1964, at least 10 years before diagnosis, in 265 companies in which exposure to formaldehyde was identified. The results do not support the hypothesis that formaldehyde is associated with lung cancer (SPIR = 1.0, 410 cases). Significantly elevated risks were found for cancers of the colon (SPIR = 1.2, 166 cases), kidney (SPIR = 1.3, 60 cases), and sino-nasal cavities (SPIR = 2.3, 13 cases). For sino-nasal cancer, a relative risk of 3.0 (95 percent confidence interval = 1.4-5.7) was found among blue-collar workers with no probable exposure to wood dust, the major confounder. This study provides further evidence that occupational exposure to formaldehyde increases the risk for sino-nasal cancer.