Although immunologic mechanism including complement activation as a pathophysiology of Korean hemorrhagic fever has been suggested, measurable complement activation resulting in the formation of anaphylatoxins, C3a, C4a and C5a, has not been verified to date. The purpose of this study was to determine if biologically active products of complement appear during the course of Korean hemorrhagic fever, and to investigate which of the two complemene activation pathways, classic or alternative, is activated. The levels of C3a C4a and C5a in serial plasma samples obtained from 15 patients with Korean hemorrhagic fever were measured by radioimmunoassay. The mean plasma C3a concentration, as well as C4a and C5a, in patients was significantly higher than normal control during all phases of Korean hemorrhagic fever. The mean plasma C level was highest in patients in the febrile and convalescent phase and this phenomenon was consistently observed with C4a and C5a. Poor correlations were observed between the levels of C3a, C4a and C5a, respectively. It was concluded that complement activation undoubtedly occurred in Korean hemorrhagic fever, and this strongly suggested that biologically active products of complement were generated during the enormous activation of the complement system via both the classic and alternative pathways.