BACKGROUND: The aims of this study are to investigate the impact of pre-existing diabetes and diabetes treatments on lung cancer prognosis. METHODS: A total of 2484 women with confirmed incident lung cancer from the Women's Health Initiative were followed for an average of 2.9 years through the date of death or 29 August 2014. RESULTS: Compared with women with lung cancer but without diabetes, women with lung cancer and diabetes had significantly increased risk of overall mortality (HR=1.27, 95% CI: 1.07-1.50). Women with diabetes receiving insulin or metformin or women who had long duration of diabetes also had increased risk of overall mortality. CONCLUSIONS: Our large prospective study provides evidence that pre-existing diabetes is associated with poor overall survival among women with lung cancer, but do not support the hypothesis that metformin use may have a protective effect in women with lung cancer and diabetes.