Airway wall remodeling is an established pathological feature in asthma. Its causes are not well understood, but one mediator of potential relevance is transforming growth factor-beta 1 (TGF- β 1). We have measured levels of immunoreactive TGF- β 1 in bronchoalveolar lavage (BAL) fluid from clinically stable atopic asthmatics and healthy control subjects. We have also examined the influence of allergen exposure on TGF- β 1 release in the airways using a segmental bronchoprovocation model, with BAL performed at two time points following endobronchial allergen and sham saline challenges. Basal concentrations of TGF- β 1 were significantly higher in asthmatics than control subjects (median 8.0 versus 5.5 pg/ml, p = 0.027). Following segmental bronchoprovocation, concentrations of TGF- β 1 at the allergen- and saline-challenged sites were not significantly different after 10 min, (31.3 versus 25.0 pg/ml, p = 0.78), but after 24 h there were significantly higher TGF- β 1 concentrations at the allergen-challenged sites (46.0 versus 21.5 pg/ml, p = 0.017). We conclude that basal TGF- β 1 levels in the airways are elevated in atopic asthma and that these levels increase further in response to allergen exposure. These findings are consistent with the hypothesis that TGF- β 1 is implicated in airway wall remodeling in asthma.