The possible influence of dietary antioxidants, especially vitamin C, on the increasing prevalence of asthma is explored. Vitamin C intake in the general population appears to correlate with asthma, suggesting that a diet low in vitamin C is a risk factor for asthma. Epidemiological studies show associations among oxidant exposure, respiratory infections, and asthma in children of smokers. Symptoms of ongoing asthma in adults appear to be increased by exposure to environmental oxidants and decreased by vitamin C supplementation. There is evidence that oxidants produced endogenously by overactive inflammatory cells contribute to ongoing asthma. Vitamin C is the major antioxidant substance present in the airway surface liquid of the lung, where it could be important in protecting against both endogenous and exogenous oxidants. Nitrogen oxides are exemplary of oxidants that could arise from both endogenous and environmental sources, which are protected against by vitamin C, and that may be important in causation and propagation of asthma.