The cytokine network is activated in the pathogenetic mechanisms of septic diseases. We measured serum concentrations of interleukin-8 (IL-8) in 152 patients with septic syndrome and 25 control subjects. IL-8 concentration in patients with septic syndrome was significantly (P<.05) higher than in controls. Serum IL-8 concentration was assayed prospectively in 6 patients undergoing chemotherapy for leukemia. Increased IL-8 concentration preceded onset of fever and elevation of C-reactive protein by 1 day. Human peripheral whole blood cells were stimulated with phorbol 12–myristate 13–acetate (PMA), lipopolysaccharide (LPS), or lipoteichoic acid. IL-8 mRNA expression was detected 10 minutes after PMA or LPS exposure, and IL-8 was secreted into culture media 3 hours after exposure. These data suggest that IL-8 is a useful early indicator of bacterial infection.