The relationship between incidence of lung cancer and intake of dietary fats, high-fat foods, fish, and fish products was studied in 25,956 men and 25,496 women aged 16-56 years attending Norwegian health screening between 1977 and 1983. Linkage to the Cancer Registry of Norway and the Central Bureau of Statistics of Norway ensured a complete follow-up until 31 December 1991. Diet was recorded on a semiquantitative food-frequency questionnaire, and 153 cases of lung cancer were identified during follow-up. Mean age at diagnosis was 56 years. After adjusting for smoking status, gender, age at screening, and attained age, significant lower risks were found for cod liver oil supplement (incidence rate ratio (IRR) = 0.5, 95% CI = 0.3-1.0) and for skim milk compared to whole milk (IRR = 0.5, 95% CI = 0.3-0.9)). No association was found with dietary cholesterol and saturated fat. A threshold of increased risk starting at the second quartile was seen for mono- and polyunsaturated fat. The potential protective effect of cod liver oil, a supplement rich in preformed vitamin A, omega-3 fatty acids, and monounsaturated fat, has to our knowledge, not been reported before. Confounding from lifestyle is possible, but the result deserves further investigation.