We attempted to determine whether persons susceptible to heatstroke produced higher serum concentrations of interleukin-6, tumor necrosis factor-alpha, and interleukin-10 when subjected to heat stress. Nine patients with previous heatstroke and 21 matched controls were subjected to a heat-stress test (at 32 degrees C, 67% humidity, 900 W/m2 solar radiation, and wind speed of 2.5 m/s) for 60 minutes and rested at 24 degrees C for another 60 minutes in full battle gear. During the heat-stress test, blood was drawn at intervals of 0, 30, 60, and 120 minutes, and serum lipopolysaccharide, interleukin-6, tumor necrosis factor-alpha, and interleukin-10 concentrations were measured. Patients with previous heatstroke had a higher mean core temperature (0.6 degree C; p < 0.05) and sweated less (0.3 liters; p < 0.05). During the heat-stress test, the lipopolysaccharide levels were not increased and there was no difference in the serum cytokine concentrations between patients with previous heatstroke and controls. However, patients with previous heatstroke had higher absolute serum cytokine concentrations and poorer thermoregulatory response to heat stress in terms of core temperature and sweat loss.