Rationale: Vitamin D deficiency is associated with chronic lung disease. We have previously shown in an in vivo mouse model that maternal vitamin D deficiency is associated with alterations in early life lung structure and function. However, there are limited data to support a relationship between maternal vitamin D deficiency during the early stages of lung development and postnatal lung function in human populations. Objectives: To assess the association between maternal vitamin D deficiency, postnatal lung function, and asthmatic status in a longitudinal birth cohort. Methods: Serum was collected at 16 to 20 weeks' gestation at the time of recruitment in a community-based prospective birth cohort for measurement of vitamin D (25[OH]D). Lung function was assessed by spirometry according to American Thoracic Society guidelines in children at 6 and 14 years of age. Demographic and clinical history data were collected by questionnaire at recruitment and at the follow-up visits. Measurements and Main Results: FVC Z-scores in both sexes (β, 0.007 [95% confidence interval (CI), 0.001-0.013]; P = 0.02) and FEV1 Z-scores in girls (β, 0.007 [95% CI, 0.001-0.013]; P = 0.02) were positively associated with maternal serum 25(OH)D at 6 years of age. These associations were mostly absent at 14 years of age. Maternal vitamin D deficiency was positively associated with asthma at 6 years of age but only in boys (odds ratio, 3.03 [95% CI, 1.02-9.02]; P = 0.04). Conclusions: This study supports the notion that vitamin D deficiency during lung development may impact on postnatal lung growth and increase the risk of developing lung disease. Copyright © 2014 by the American Thoracic Society.