Epidemiological studies have established a relationship between low levels of serum vitamin D and reduced lung function in healthy adults, and asthma onset and severity in children. However, no study has examined the relationship between vitamin D levels and exercise-induced bronchoconstriction in asthmatic children. We evaluated the relationship between 25-hydroxyvitamin D concentrations and baseline forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1) and change in FEV1 (ΔFEV1) after a standardised exercise challenge in 45 children with intermittent asthma. Only 11% of the children had desirable serum vitamin D levels (at least 30-40 ng·mL(-1)). A positive correlation was found between serum 25-hydroxyvitamin D and both FVC (r=0.34; p=0.037) and FEV1 (r=0.32; p=0.037). Subjects with a positive response to the exercise challenge (ΔFEV1≥10%) presented lower serum levels of 25-hydroxyvitamin D than children with a negative challenge (mean±sd 16.2±5.2 versus 23.4±7.0 ng·mL(-1), respectively; p=0.001). Our results indicate that hypovitaminosis D is frequent in asthmatic children who live in a Mediterranean country. In those children, lower levels of vitamin D are associated with reduced lung function and increased reactivity to exercise.