Forty critically ill surgical patients with documented infections were studied during their stay in an intensive care unit. Among these patients, 19 developed septic shock and 16 died, 9 of them from septic shock. Interleukin 1β (IL-1β), tumor necrosis factor (TNFα), and interleukin 6 (IL-6) were measured each day and every 1 or 2 hours when septic shock occurred. Although IL-1β was never found, TNFa was most often observed in the serum at a level under 100 pg/mL except during septic shock. During these acute episodes TNFa level reached several hundred pg/ mL, but only for a few hours. In contrast, IL-6 was always increased in the serum of acutely ill patients (peak to 500,000 pg/ mL). There was a direct correlation between IL-6 peak serum level and TNFa peak serum level during septic shock and between IL-6 serum level and temperature or C-reactive protein serum level. Moreover, IL-6 correlated well with APACHE II score, and the mortality rate increased significantly in the group of patients who presented with IL-6 serum level above 1000 pg/ mL. Thus, IL-6 appears to be a good marker of severity during bacterial infection.