Constitutional reaction to a single sting of an hymenopteron occurred in 400 unselected patients. The average patient had a family history of allergy and a personal history of previous insect stings. Eight percent of patients, however, had no previous warning of anaphylaxis, and 6% received intravenous stings. Epinephrine hydrochloride was the drug of choice in the crisis. Of the patients who had received hyposensitizing treatments, 88% were protected against adverse reactions to subsequent stings.