The activities (C3a, C4a, C5a) and concentrations (CH50, C3, C4, C5) of serum complement were measured to evaluate the involvement of complement in sepsis. We studied 27 patients with sepsis who were divided into the survivors (Group 1) and the nonsurvivors (Group 2). The levels of C3a, C4a, and C5a were all significantly higher in Group 2 than in Group 1 and closely reflected the severity of sepsis. Levels of C3 and C4 were significantly lower in Group 2 than in Group 1, presumably because these components were consumed as a result of activation of both the alternative and classical pathways. The levels of CH50 and C5 did not differ significantly between the two groups. These findings suggest that complement is closely involved in the exacerbation of the condition of septic patients, and that the measurement of complement activity is useful for evaluating the severity of sepsis.