Thirty-one patients, aged 6 to 78 years, with anaphylactoid reactions of unknown cause were retrospectively evaluated. All of the patients had life-threatening hypotension, wheezing or upper airway obstruction, and simultaneous angioedema or urticaria. Twenty-six patients also had episodes of angioedema or urticaria alone. In nine patients the initial episode was the most severe. In 16 of 18 patients, medications were effective in controlling or reducing the severity of the symptoms. In ten patients, the reactions were always self-limited and no medications were needed. No deaths occurred in 210 anaphylactoid attacks during 164 patient-years. Remissions lasting more than one year occurred in seven patients. Laboratory studies, food diaries, and skin tests were not helpful in establishing an etiology, although atopy was present in 12 of 15 patients tested.