A study of 759 histologically verified cancers of the nasal cavity (287 cases), paranasal sinuses (179 cases), and nasopharynx (293 cases) and 2465 cancer controls diagnosed in Denmark between 1970 and 1982 was conducted to investigate the importance of occupational exposure to formaldehyde. Information on job history for cases and controls was derived from a national data linkage system and exposure to formaldehyde and wood dust was assessed by industrial hygienists unaware of the case-control status of the patients. The exposure rates for formaldehyde among male and female controls were 4.2% and 0.1%, respectively. After proper adjustment for contemporary wood dust exposure, relative risks of 2.3 (95% CI = 0.9-5.8) for squamous cell carcinoma and 2.2 (95% CI = 0.7-7.2) for adenocarcinoma of the nasal cavity and paranasal sinuses were detected among men who have ever been exposed to formaldehyde in their job compared with those never exposed. The introduction of a 10 year latency period did not change the risk estimates substantially. It was considered unlikely that the results were due to bias or misclassification of exposure although the effect of chance could not be excluded.