Background: Studies have suggested that physical activity may decrease cancer incidence and mortality. Data fromthe Women’s Health Initiative Observational Study (WHI-OS) and Clinical Trial (WHI-CT) were used to investigate this relationship in a prospective cohort of postmenopausal women. Methods: The WHI study enrolled women aged 50-79 years from 1993-1998 at 40 U.S. clinical centers. Among 134,058participants, Cox proportional hazards models were used to assess the association between lung cancer incidence/mortality and physical activity levels at study baseline [MET-minutes/week: inactive < 100 (reference), low 100- < 500, medium 500- < 1200, high 1200+] and sedentary behavior (sitting time/day in WHI-OS). Results: Over 11.8 mean follow-up years, 2,229 total incident lung cancer cases and 1,427 lung cancer deaths were identified. Increased total physical activity at study entry was associated with significant decreases in both lung cancer incidence [p = 0.004; physical activity levels: low HR 0.86 (95% CI: 0.77-0.96), medium HR 0.81 (0.72-0.91), high HR 0.89 (0.78-1.01)] and mortality [p < 0.0001, low HR 0.80 (0.70-0.92), medium HR 0.68 (0.58-0.79), high HR 0.77 (0.65-0.91)]. BMI was found to be an effect modifier of the relationship between total physical activity and lung cancer incidence (p = 0.008), with reduced HRs found for increasing physical activity levels for BMI < 30. While smoking status was not a statistically significant effect modifier, former smokers > 10 pack-years and current smokers were estimated to have reduced lung cancer incidence and mortality HRs for increasing physical activity levels, when compared to inactive women with the same smoking status. In the subtype analysis, decreased lung cancer mortality was associated with increased total physical activity levels for both overall NSCLC and adenocarcinoma. No association was found for sedentary behavior and lung cancer incidence or mortality. Conclusions: In a cohort of postmenopausal women, physical activity prior to diagnosis may decrease lung cancer incidence and mortality. Physical activity may be particularly protective for women with BMIs under 30, and for current and heavy former smokers. Clinical trial information: NCT00000611