Background: Smoking and severe asthma exacerbations in pregnancy are risk factors for low birth weight babies. No studies have assessed the clinical implications of smoking on asthma exacerbations in pregnancy. Methods: Pregnant women with current asthma (n=80) were prospectively assessed at clinic visits (18, 30, 36 weeks), during exacerbations and with fortnightly phone calls. The asthma control questionnaire was administered at each contact and exacerbations classified as severe (requiring medical intervention) or mild (self-managed). Medications, self-management skills, smoking history, fractional exhaled nitric oxide (FENO), exhaled carbon monoxide (ECO) and lung function were assessed. Pregnant women without asthma (controls, n=46) were assessed prospectively at clinic visits. Results: Women with asthma were more likely to smoke (34% current smokers) than women without asthma (15% current smokers). In women with asthma, the median (IQR) exacerbation rate during pregnancy was 2.0 (1.0-3.0) in current smokers, 2.0 (1.0-3.0) in ex-smokers and 1.5 (1.0-2.0) in never smokers. The asthma control score during exacerbations was higher in current smokers (median (IQR) 2.17 (1.17-2.7)) compared with never smokers (1.17 (0.8-2.17), p=0.056). An adjusted linear regression model found that smoking was significantly associated with higher asthma control score during exacerbation (p=0.04). Birth weights were lower among children of smokers than non-smokers (p=0.023 control group, p=0.086 asthma group). Conclusions: During pregnancy, asthma exacerbations are more common and more severe in current smokers than never smokers. The risk of effects of maternal asthma on the fetus may be greater among smokers.