• One hundred cases of heatstroke were selected for analysis from more than 1,000 hospital admissions during three exceptionally hot summers in a large city. These patients had the hot, dry skin that distinguishes heatstroke from other syndromes caused by high environmental temperatures. They also had symptoms referable to the central nervous system and temperatures usually exceeding 41.1 C (106 F). The greatest risk of life was in those patients with the severest central nervous system abnormalities; there were 12 deaths among the 53 patients who were comatos when admitted. Temperatures over 41.1 C, uremia, hypotension, and hyperpotassemia were also poor prognostic signs; all of the four patients with hyperpotassemia died. Treatment began with prompt total immersion and vigorous massage in an ice bath; this was followed by attention to cardiovascular status, electrolyte-fluid balance, oxygen requirements, and concurrent disease. Some of the data from laboratory studies on blood and cerebrospinal fluid supplied valuable therapeutic guidance, and a comparison with past mortality figures indicated that both prophylaxis and treatment have become more effective.