Allergic asthma is a chronic disease of the airways associated with airway hyperresponsiveness, a variable degree of airflow obstruction, airway remodelling and a characteristic airway inflammation. Factors of the vitamin D axis, which include vitamin D metabolites and vitamin D binding protein (VDBP), have been linked to asthma, but only few data exist about their regulation in the lung during acute allergen-induced airway inflammation. Therefore, we analysed the regulation of factors of the vitamin D axis during the early- and late-phase reaction of allergic asthma. Fifteen patients with mild allergic asthma underwent segmental allergen challenge. VDBP was analysed in bronchoalveolar lavage fluid (BALF) and serum using the enzyme-linked immunosorbent assay (ELISA) technique. 25-hydroxyvitamin D(3)[25(OH)D(3)] and 1,25-dihydroxyvitamin D(3)[1,25(OH)(2)D(3)] were analysed by a commercial laboratory using the liquid chromatography-mass spectrometry (LC/MS) technique. VDBP (median 2·3, range 0·2-7·1 μg/ml), 25(OH)D(3) (median 0·060, range < 0·002-3·210 ng/ml) and 1,25(OH)(2)D(3) (median < 0·1, range < 0·1-2·8 pg/ml) were significantly elevated in BALF 24 h but not 10 min after allergen challenge. After correction for plasma leakage using the plasma marker protein albumin, VDBP and 25(OH)D(3) were still increased significantly while 1,25(OH)(2)D(3) was not. VDBP and 25(OH)D(3) were correlated with each other and with the inflammatory response 24 h after allergen challenge. Serum concentrations of all three factors were not influenced by allergen challenge. In conclusion, we report a significant increase in VDBP and 25(OH)D(3) in human BALF 24 h after allergen challenge, suggesting a role for these factors in the asthmatic late-phase reaction.