Didymella exitialis and late summer asthma

4 patients in whom Didymella exitialis seemed to be an important cause of their asthma were investigated. All 4 patients had immediate skinprick test reactions and specific IgE antibody to D exitialis and Alternaria alternata. Inhalation of a soluble extract of D exitialis provoked immediate asthmatic reactions in these 4, but not in a skin-prick-test-negative volunteer. Radioallergosorbent inhibition studies with D exitialis and A alternata showed self and cross inhibition, which suggested shared antigens. During summer, 1983, 1 of the 4 recorded peak expiratory flow rate (PEFR) twice a day. Spore counts of D exitialis and A alternata were made during this time. Falls in PEFR occurred when D exitialis spore counts increased, usually after a rainfall. Of 34 allergy clinic patients, 6 had skin-prick test reaction to both fungi, but 2 had reactions to A alternata only and 3 to D exitialis only, which suggests the existence of specific antigens. D exitialis seems to be a cause of late summer asthma. Its importance is not known, but allergy to it may contribute to asthma following thunderstorms.