A hospital-based case-control study among non-smoking women was conducted to clarify risk factors in non-smoking females in Japan. Cases consisted of 144 non-smoking female lung cancer patients, and these were compared to 713 non-smoking female controls. The odds ratio (95% confidence interval) for use of wood or straw as cooking fuels when subjects were 30 years old was estimated as 1.77 (1.08 to 2.91). For those whose household members, other than husbands, had smoked, the odds ratio was estimated as 1.50 (1.01 to 2.32). For those whose mothers had smoked, the odds ratio was estimated as 1.28 (0.71 to 2.31). Use of heating appliances did not show an elevated risk. Some points to be noted in the study of low-risk agents for lung cancer are discussed.