We performed a prospective study of anaphylaxis in 25 consecutive patients. Three distinct clinical patterns were observed: uniphasic, biphasic, and protracted anaphylaxis. Thirteen patients (52%) experienced a single episode. Biphasic anaphylaxis occurred in five patients (20%), two episodes of hypotension or laryngeal edema separated by asymptomatic intervals of 1 to 8 hours. Initial therapy included large doses of glucocorticoids in three of the five patients. Seven patients (28%) suffered hypotension, lower respiratory obstruction, or laryngeal obstruction that persisted 5 to 32 hours despite vigorous therapy that included systemic glucocorticoids. Recurrent or prolonged reactions were 2.8-fold more likely if the onset was 30 or more minutes after exposure to the stimulus or if the offending agent had been administered by mouth (p less than 0.03). Fatal reactions occurred in one patient who was taking propranolol and in one who was adrenal insufficient. The results of this study demonstrate that biphasic and protracted anaphylaxis are common, despite glucocorticoid therapy. Potentially life-endangering recurrences occurred in five (28%) of the 18 patients who responded to initial therapy. These observations indicate that patients should be followed carefully after apparent remission of anaphylaxis.