The involvement of C-reactive protein (CRP) in the activation of complement in patients with sepsis was investigated. In 104 patients with infections of varying severity, circulating levels of CRP-complement complexes, which are specific indicators for CRP-mediated complement activation, were assessed. Complement-CRP complexes were increased in almost all patients and correlated significantly with levels of C3a (r = .59; P < .001) and C-reactive protein (r = .76; P < .001). In addition, they correlated with levels of secretory phospholipase A2 (r = .59; P < .001). Levels of complement-CRP complexes in patients with a pneumococcal type of infection were similar to those in patients with other types of infections. Complement-CRP complexes were significantly higher in patients with shock (P = .01) and in patients who died (P = .03). These results demonstrate that part of the complement activation in patients with sepsis is independent from a direct interaction with microorganisms but rather results from an endogenous mechanism involving CRP.