Abstract Rationale: Evidence suggests that the effects of smoking cannabis on lung function are different from tobacco. However, long-term follow-up data are scarce and mostly based on young adults. Objectives: To assess the effects of cannabis and tobacco on lung function in mid–adult life. Methods: Cannabis and tobacco use were reported at ages 18, 21, 26, 32, 38, and 45 years in a population-based cohort study of 1,037 participants. Spirometry, plethysmography, and carbon monoxide transfer factor were measured at age 45. Associations between lung function and cannabis use were adjusted for tobacco use. Measurements and Main Results: Data were available from 881 (88%) of 997 surviving participants. Cumulative...