Vitamin C (500 mg·day-1) supplementation for pregnant smokers has been reported to increase newborn pulmonary function and infant forced expiratory flows (FEFs) at 3 months of age. Its effect on airway function through 12 months of age has not been reported.To assess whether vitamin C supplementation to pregnant smokers is associated with a sustained increased airway function in their infants through 12 months of age.This is a prespecified secondary outcome of a randomised, double-blind, placebo-controlled trial that randomised 251 pregnant smokers between 13 and 23 weeks of gestation: 125 to 500 mg·day-1 vitamin C and 126 to placebo. Smoking cessation counselling was provided. FEFs performed at 3 and 12 months of age were analysed by repeated measures analysis of covariance.FEFs were performed in 222 infants at 3 months and 202 infants at 12 months of age. The infants allocated to vitamin C had significantly increased FEFs over the first year of life compared to those allocated to placebo. The overall increased flows were: 40.2 mL·sec-1 for FEF75 (adjusted 95% CI for difference 6.6 to 73.8; p=0.025); 58.3 mL·sec-1 for FEF50 (95% CI 10.9 to 105.8; p=0.0081); and 55.1 mL·sec-1 for FEF25-75 (95% CI, 9.7 to 100.5; p=0.013).In offspring of pregnant smokers randomised to vitamin C versus placebo, vitamin C during pregnancy was associated with a small but significantly increased airway function at 3 and 12 months of age, suggesting a potential shift to a higher airway function trajectory curve. Continued follow-up is underway.