Heat stroke is the most serious entity amongst the heat disorders. Whilst potentially fatal, it is preventable and salvageable. A retrospective study involving 27 patients admitted for exertional heat stroke to the Medical Unit, Toa Payoh Hospital, Singapore from January, 1984 to January, 1987 was carried out. These patients presented with a rectal temperature of greater than 40 degrees C and central nervous system disturbances. All were males and, except for three, were local born. All except two were below thirty years old. The patients were treated with a standard regime of IV fluids and sponging in the ICU. 19 patients (70.4%) presented in coma whilst abnormal behaviour, e.g. aggression and mental confusion was seen in the remainder. Fits were seen in only 5 patients (18.5%). Metabolic acidosis was seen in 93.3%. Hypokalaemia was present in 3 patients (11.1%). Of the enzymes, creatinine phosphokinase was elevated in all except 1 patient (mean: 4868.8, range: 146-28850). There were no deaths recorded in this series. Complications include reversible DIVC (3 instances), oliguric renal failure (4 instances) and residual neurological deficit (2 instances). 9 patients (39.1%) took more than 4 hours for the rectal temperature to attain 37.5 degrees C. All the above complications except for 1 instance of DIVC occurred in this group with delayed cooling. Heat stroke is a serious condition with serious complications and require prompt treatment.