Thyrotoxic crisis ("thyroid storm"), even with improved methods of treating hyperthyroidism, is by no means an extinct phenomenon. Of a total of 2,033 thyrotoxic patients admitted to the wards of the Massachusetts General Hospital in the past twenty-five years, 36 have experienced a toxic crisis. We have thought it worth while to place the analysis of these cases on record since so few1 of the opinions expressed in the literature of thyroid storm are based on experience with large series of patients. Twenty-five of these storms (hereinafter called "surgical") have followed the 1,383 operations performed on these patients; the remaining 11 storms (hereinafter called "medical") have represented an acute breakdown of thermal regulation in patients suffering from fulminating thyrotoxicosis. Eight of the medical and 16 of the surgical storms resulted in the death of the patient. In our view thyroid crisis is not strictly a complication of thyrotoxicosis but