We reviewed the clinical and laboratory characteristics of 34 patients who had classical heatstroke during the Kansas City heat wave of 1980. The patients were elderly, predominantly black, and of low socioeconomic class. Overall mortality was 18%, with 9% of patients exhibiting severe residual neurologic deficit; 73% had full recovery. Patients with coma, temperature greater than or equal to 108 F (42.2 C), severe hypotension, coagulopathy, and need for respiratory assistance were at highest risk of death. Associated disease was common (67%), with hypertension (32%), diabetes (21%), and alcoholism (21%) being most frequent. Medications known to predispose to heatstroke were used by 56% of patients. Hematologic abnormalities were nonspecific, and clinical evidence of renal or hepatic failure was rare. Hyponatremia, hypokalemia, hypocalcemia, hypomagnesemia, hypophosphatemia, and elevated levels of creatine phosphokinase and glucose were frequent but did not correlate with outcome. The predominant arterial blood gas abnormality was metabolic acidosis or a combined metabolic acidosis and respiratory alkalosis.