We examined the characteristics of allergen-induced inflammation of the bronchial mucosa in asthmatic patients. Studies were carried out 4 h (eight patients) and 24 h (nine patients) after allergen inhalation challenge; 10 patients were not challenged and served as control subjects. We found that in the control group the ratio of degranulating to granulated mast cells was higher in patients with than in patients without late-phase response. In patients studied 4 h after allergen challenge the total number of mast cells was not significantly different from that in control subjects; the ratio of degranulating to granulated mast cells was increased similarly in patients with and without late-phase response. Among patients studied 24 h after allergen challenge, those who had developed the late-phase response had an increased (p less than 0.05) number of mast cells as compared with patients who had not developed the late-phase response, the number of mast cells was significantly correlated with the severity of the late-phase response (r = 0.80; p less than 0.001). The numbers of eosinophils and mononuclear cells and the morphologic abnormalities of bronchial structure (altered ratio of cylindrical to goblet cells, thickening of the basement membrane, and edema and angiectasis of lamina propria) were similar in the different groups of patients. We conclude that the inflammatory events leading to the development of the late-phase asthmatic response to allergen represent a stimulus for an increase in the number of mast cells in the bronchial mucosa.