This study evaluates the histological changes, especially the presence of possible precancerous lesions, in the nasal mucosa ofworkers 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.The inhalation of formaldehyde has been shown to induce squamous cell carcinoma in the nasal cavity of rats at exposure levels ranging from 6 to 14 3 ppm12 and mice at an exposure level of 14 3 ppm.3 Although these findings aroused considerable concern about the possible long term effects in man, epidemiological studies conducted to date have not provided unequivocal evidence that exposure can induce nasal or other types of cancer.45 Like most other types of epithelial malignancies, it is reasonable to assume that nasal cancer is preceded by preneoplastic states and that the prevalence of such lesions is considerably higher than that of cancer. Experimental studies have shown that animals exposed to formaldehyde concentrations of 2 ppm or more develop dysplasia and metaplasia of