Background: Mast cells are versatile effector cells of primary importance in asthma and airway inflammation. During inflammation mast cells accumulate in the bronchial epithelium. The mechanism for this increase in mast cell number has not been defined. Objectives: The aim of this study was to examine the presence of mast cell chemotactic activity in bronchoalveolar lavage (BAL) fluid taken before and at the end of 2 pollen seasons from patients with allergic asthma. Methods: To measure mast cell chemotactic activity, we used a modified Boyden chamber and the human mast cell line HMC-1 or in vitro–developed mast cells as responder cells. Results: A total of 27 patients were investigated, of which 8 exhibited mast cell chemotactic activity in their BAL fluid collected before season. A significant increase in the activity was found in 18 of 27 BAL fluids sampled at the end of the pollen season. No difference was found between patients treated with immunotherapy or placebo. The presence of stem cell factor could be detected in all BAL fluids analyzed. Blocking antibodies against stem cell factor or transforming growth factor-β partially blocked the activity in some of the BAL fluids. Treatment of the responder cells with pertussis toxin reduced the migratory activity in 13 of 14 BAL fluids collected during pollen season. Conclusion: This study demonstrates the presence of mast cell chemotactic activity in BAL fluids from patients with allergic asthma, with a significant increase in activity during pollen season. The major part of this activity consisted of factors mediating their effect through G_i-protein coupled receptors. This activity may be responsible for the mast cell accumulation in the intraepithelial layer seen in allergic asthmatic patients. (J Allergy Clin Immunol 2000;105:455-61.)