OBJECTIVE: Epidemiological evidence of diabetes as a lung cancer risk factor is limited and conflicting. Therefore, we assessed associations among diabetes, diabetes therapy, and lung cancer risk in postmenopausal women participating in the Women's Health Initiative (WHI) study. RESEARCH DESIGN AND METHODS: Postmenopausal women (n = 145,765), ages 50-79 years, including 8,154 women with diabetes at study entry were followed for a mean of 11 years with 2,257 lung cancers diagnosed. Information on diabetes therapy was collected via two methods (self-reported information on treatment history collected on a questionnaire at baseline and a face-to-face review of current medication containers that participants brought to the baseline visit). Lung cancers were confirmed by central medical record and pathology report review. Cox proportional hazards regression models adjusted for lung cancer risk factors were used to estimate hazard ratios (HRs) (95% CI) for diagnosis of diabetes and treatment of disease as risk factors for lung cancer. RESULTS: Compared with women without diabetes, women with self-reported treated diabetes had a significantly higher risk of lung cancer (HR 1.27 [95% CI 1.02-1.59]), with risks increasing for women with diabetes requiring insulin treatment (1.71 [1.15-2.53]). However, we did not observe a significant association between lung cancer risk and diabetes not treated with medication or with duration of diabetes. CONCLUSIONS: Postmenopausal women with treated diabetes, especially those using insulin, have a significantly higher risk of lung cancer. The influence of diabetes severity and specific classes of therapy for diabetes on lung cancer risk require future study.