As first described by Stark and Sullivan and reported subsequently by others, anaphylaxis may present as a biphasic syndrome with recurrences several hours after apparent resolution of the initial signs and symptoms. Biphasic anaphylaxis has been reported in between 4% and 20% of cases. Signs and symptoms experienced during the recurrent phase may be equivalent to, or worse than, those associated with the initial reaction. A protracted (persistent) syndrome may also occur and may not respond to treatment with glucocorticosteriods. Since life-threatening manifestations may recur, it may be necessary to observe patients for up to 12 hours after apparent recovery from an anaphylactic episode.