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Introduction: Smoking increases the risk of asthma and reduces lung function among subjects with and without asthma. We assessed the effects of smoking on lung function reflecting both central and small airways among adults with newly onset asthma., Methods: In a population-based study, 521 (response rate 86%) working-aged adults with clinically defined newly diagnosed asthma answered a questionnaire on personal smoking and other factors potentially influencing lung function, and performed spirometry. We applied multiple linear regression analysis to estimate the relations between smoking and lung function adjusting for confounding., Results: Among asthmatics, FEV 1 level was reduced significantly, on average 208 mL, related to regular smoking (adjusted effect estimate -0.208, 95% CI -0.355 to -0.061) and 245 mL in relation to former smoking, that is, among those who quit less than a year ago (-0.245, 95% CI -0.485 to -0.004). In contrast, FEV 1 was not significantly related to occasional smoking or former smoking among those who quit over a year ago. Forced expiratory flow (FEF) levels (L/s) were also significantly reduced among regular smokers (FEF 25-75% : -0.372, 95% CI -0.607 to -0.137; FEF 50% : -0.476, 95% CI -0.750 to -0.202). An exposure-response pattern related to both daily smoking rate and lifetime cumulative smoking was seen both among men and women., Conclusions: This study provides new evidence that among working-aged adults with new asthma, regular smoking and former smoking reduce lung function levels with a dose-response pattern. The lung function parameters applied as outcomes reflect both larger and smaller airways., Competing Interests: Competing interests: None declared.