In order to obtain information about the nature of the local inflammatory process during late asthmatic reactions after house dust mite inhalation, bronchoalveolar lavage (BAL) was performed in 19 asthmatic patients and in 5 control subjects. In 16 of the patients and in all of the control subjects, BAL was performed 6 to 7 h after allergen inhalation. Six of the patients showed early and late asthmatic reactions (LAR), 5 showed early reactions, and 5 showed no reactions. Bronchoalveolar lavage was also performed shortly after the early reaction in 5 patients with documented combined early and late reactions. In the BAL fluid of the patients with LAR, a significant eosinophilia (0.01 less than p less than 0.05) was found compared with that in all other patient groups and with that in the control subjects. This bronchoalveolar eosinophilia was accompanied by elevated eosinophil cationic protein/albumin ratio in the BAL fluid (0.01 less than p less than 0.05). These observations suggest that eosinophils and their mediators might be involved in the development of LAR after allergen inhalation.