Context: Marijuana smoke contains many of the same constituents as tobacco smoke, but whether it has similar adverse effects on pulmonary function is unclear. Objective: To analyze associations between marijuana (both current and lifetime exposure) and pulmonary function. Design, Setting, and Participants: The Coronary Artery Risk Development in Young Adults (CARDIA) study, a longitudinal study collecting repeated measurements of pulmonary function and smoking over 20 years (March 26, 1985-August 19, 2006) in a cohort of 5115 men and women in 4 US cities. Mixed linear modeling was used to account for individual age-based trajectories of pulmonary function and other covariates including tobacco use, which was analyzed in parallel as a positive control. Lifetime exposure to marijuana joints was expressed in joint-years, with 1 joint-year of exposure equivalent to smoking 365 joints or filled pipe bowls. Main Outcome Measures: Forced expiratory volume in the first second of expiration (FEV 1) and forced vital capacity (FVC). Results: Marijuana exposure was nearly as common as tobacco exposure but was mostly light (median, 2-3 episodes per month). Tobacco exposure, both current and lifetime, was linearly associated with lower FEV 1 and FVC. In contrast, the association between marijuana exposure and pulmonary function was nonlinear (P<.001): at low levels of exposure, FEV 1 increased by 13 mL/joint-year (95% CI, 6.4 to 20; P<.001) and FVC by 20 mL/joint-year (95% CI, 12 to 27; P<.001), but at higher levels of exposure, these associations leveled or even reversed. The slope for FEV 1 was -2.2 mL/joint-year (95% CI, -4.6 to 0.3; P=.08) at more than 10 joint-years and -3.2 mL per marijuana smoking episode/mo (95% CI, -5.8 to -0.6; P=.02) at more than 20 episodes/mo. With very heavy marijuana use, the net association with FEV 1 was not significantly different from baseline, and the net association with FVC remained significantly greater than baseline (eg, at 20 joint-years, 76 mL [95% CI, 34 to 117]; P<.001). Conclusion: Occasional and low cumulative marijuana use was not associated with adverse effects on pulmonary function. ©2012 American Medical Association. All rights reserved.