This study evaluates the histological changes, especially the presence of possible precancerous lesions, in the nasal mucosa of workers exposed to formaldehyde. Nasal biopsies of 37 workers occupationally exposed to formaldehyde for more than five years and 37 age matched referents showed a higher degree of metaplastic alterations in the former group. In addition, three cases of epithelial dysplasia were observed among the exposed. These results indicate that formaldehyde may be potentially carcinogenic to man. Combination of this finding with the inconclusive epidemiological studies suggests that formaldehyde is a weak carcinogen and that occupational exposure to formaldehyde alone is insufficient to induce nasal cancer.