Ehrlichia sennetsu is the causative agent of human Sennetsu ehrlichiosis. Ehrlichiosis is an acute and occasionally chronic infectious disease caused by obligate intracellular bacteria in the family Rickettsiaceae. To understand the seroepidemiological patterns of ehrlichiosis in Korea, a total of 2,625 patients with acute febrile episode reported from 1990 to 1992 were surveyed using an indirect fluorescent antibody assay (IFA). The result was as follows. Seropositivity for ehrlichiosis was 3.23% by excluding highly cross-reacted sera with other rickettsial antigens. Sera reacted to E. sennetsu showed the cross reaction with other rickettsia as in the order of R. typhi 49.6%, R. conorii 31.6%, R. japonica 28.1%, C. burnetii 26.4%, R. sibirica 25.8%, O. tsutsugamushi 25.8%, R. akari 25.4%, and R. prowazekii 25.4%. Sexual difference in the seropositivity was not noted. The age groups of fifties and under the tenth showed higher prevalence than others. Seropositivity was most prevalent in July and August. As for regional distribution, Chonbuk (10.5%) showed the highest seropositive rate. Geographical distribution of the seropositivity covered most area except Cheju province in Korea.