Cigarette smoking is irrefutably the strongest risk factor for lung cancer; however, approximately 25% of cases worldwide occur among nonsmokers. The age-adjusted annual incidence rate of lung cancer in Shanghai, a region where relatively few women smoke cigarettes, is one of the highest in the world. To help further elucidate the etiology of lung cancer among nonsmokers, the authors examined hormonal factors among women who were lifetime nonsmokers. They analyzed data from the prospective Shanghai Women's Health Study, which recruited Chinese women aged 40-70 years between 1996 and 2000 from selected urban communities. The current analysis included 71,314 women (n = 220 cases) who were lifetime nonsmokers and had no history of cancer at baseline. Later age at menopause (> or =51 vs. <46 years; hazard ratio (HR) = 0.63, 95% confidence interval (CI): 0.40, 1.00), longer reproductive period (> or =36 vs. <31 years; HR = 0.60, 95% CI: 0.39, 0.93), higher parity (> or =4 vs. 0 children; HR = 0.42, 95% CI: 0.19, 0.90), and intrauterine device use (HR = 0.59, 95% CI: 0.41, 0.86) were associated with decreased risks of lung cancer. This large prospective study suggests a potential role for hormonal factors in the etiology of lung cancer among nonsmoking women.