Epidemiologic research, including our own, has demonstrated associations of arsenic (As) exposure with impaired lung function. While an overwhelming number of recent reports demonstrates a protective role of vitamin D (vit. D) on the respiratory system, its role on lung function in an As exposed population has not been evaluated. In an ongoing study in the Health Effects of Arsenic Longitudinal Study (HEALS) in Bangladesh, we have observed a positive association between serum vit. D and lung function among people exposed to As. Our data show a strong adverse association between water As and lung function among individuals with low vit. D (<=15 ng mL−1). The findings suggest that people with adequate levels of vitamin D are less likely to experience impairment of lung function associated with As. Our research has important public health implications in As exposed populations around the globe.