BACKGROUND AND OBJECTIVE: Trichophyton may be the causative allergen in patients with severe asthma. This study was performed in order to confirm Trichophyton as a causative antigen in asthmatic patients in Korea and to understand the pathogenic mechanism of Trichophyton- induced asthma. MATERIALS AND METHOD: Two groups of 33 asthmatic patients sensitized to Trichophyton anti- gen were enrolled. Group I included 12 patients showing positive results on Trichophyton bron- choprovocation test(BPT), group II included 21 patients showing negative results on Trichophyton-BPT, and group III included 19 controls showing negative results to Trichophyton antigen on skin prick test. Allergy skin prick test including Trichophyton as well as common inhalant allergens was done. To confirm bronchial sensitivity, BPT with Trichophyton extract was performed. Serum specific IgE, IgG1 and IgG4 antibodies by ELISA were done using Trichophyton mentagrophytes antigen and then compared with specific IgE results by RIA using Trichophyton tonsurans. RESULTS: Although there were no significant differences in asthmatic duration, total eosinophil count and skin reactivity(p>0.05, respectively), significant difference was noted in methacholine PC20 (p<0.05) between group I and II. There were no significant differences in specific IgE and IgG4 level between group I and group II (p<0.05). Specific IgE and IgG4 levels were significantly higher in group I than in group III (p<0.05, respectively). Two of group I patients had high specific IgG4 without specific IgE. There was no relationship between skin reactivity to Trichophyton mentagrophytes and specific IgE level. Positive predictive value of RIA for Trichophyton BPT was higher than that of ELISA. CONCLUSION: These results suggest that Trichophyton inhalation induces bronchoconstriction by IgE mediated mechanism. Trichophyton antigen should be included in skin prick test battery to screen causative agents for asthmatic patients. Further studies will be needed to evaluate the roles of specific IgG in Trichophyton-asthma.