Although smoking is a major cause of lung cancer, the proportion of lung cancer cases among Japanese women who never smoked is high. As the prevalence of smoking in Japan is relatively high in men but low in women, the development of lung cancer in nonsmoking Japanese women may be significantly impacted by passive smoking. We conducted a population-based prospective study established in 1990 for Cohort I and in 1993 for Cohort II. The study population was defined as all residents aged 40-69 years at the baseline survey. 28,414 lifelong non-smoking women provided baseline information on exposure to tobacco smoke from their husband, at the workplace and during childhood. Over 13 years of follow-up, 109 women were newly diagnosed with lung cancer, of whom 82 developed adenocarcinoma. Compared with women married to never smokers, hazard ratio (HR) [95% confidence interval (CI)] for all lung cancer incidence in women who lived with a smoking husband was 1.34 (95% CI 0.81-2.21). An association was clearly identified for adenocarcinoma (HR 2.03, 95% CI 1.07-3.86), for which dose-response relationships were seen for both the intensity (p for trend 5 0.02) and amount (p for trend 5 0.03) of the husband's smoking. Passive smoking at the workplace also increased the risk of lung cancer (HR 1.32, 95% CI 0.85-2.04). Moreover, a higher risk of adenocarcinoma was seen for combined husband and workplace exposure (HR 1.93, 95% CI 0.88-4.23). These findings confirm that passive smoking is a risk factor for lung cancer, especially for adenocarcinoma among Japanese women. ' 2007 Wiley-Liss, Inc.Key words: lung cancer; passive smoking; histological type; prospective study; Japanese non-smoking women In Japan, lung cancer has been the second leading cause of cancer death in women since the 1980s. 1 Although the majority of lung cancers can be attributed to cigarette smoking, 53% of all women with lung cancer world-wide are never smokers. 2,3 The proportion of Japanese female lung cancer patients who have never smoked is as high as 70%, 4 whereas the proportion of Japanese women aged 20 or more who smoke is only around 10%. 5 The major risk for lung cancer in Japanese women cannot therefore be attributed to smoking. Given that the urine of non-smokers exposed to passive smoking contains concentrations of carcinogenic N-nitroso compounds, which are specific to tobacco 6 and the smoking rate in Japanese men is high, at around 50%, 5 passive smoking might be an important risk factor for lung cancer in nonsmoking Japanese women.Since the publication of the first positive findings by Hirayama in Japan, many studies have investigated the relation between passive smoking and lung cancer in non-smoking women. 7 Recently, the International Agency for Research on Cancer (IARC) concluded that findings on the risk of lung cancer associated with environmental tobacco smoke were consistent. 8 Moreover, a meta-analysis of published studies estimated that the excess risk of lung cancer in non-smokers who lived with a smoker compared to those who live...