Proportionate mortality among workers exposed to formaldehyde was analyzed among employees of a large chemical plant in Western Massachusetts. Twenty-four such decedents, all males, were identified through union records, reports of former co-workers, and a systematic review of obituaries in local newspaper. Work histories were obtained from seniority lists. Race-age-sex-adjusted proportionate mortality ratios ( PMRs ) were significantly elevated for cancer of the colon based on United States, county, and county cancer mortality proportions (PMR = 702, 424, 333, p less than or equal to .05), as were PMRs for the category buccal and pharyngeal cancer (PMR = 870, 952, 833, p less than .05). This study provides evidence of formaldehyde's carcinogenicity. These findings are at variance with a previous report of the mortality experience of workers at the same plant from an earlier period.