Human ehrlichiosis is a tick-borne antropozoonosis which was described in 1987 but in Poland up to now has not been diagnosed. The purpose of our work was to present 11 cases where antibodies against Ehrlichia chaffeensis were detected in serum. In 2000 we examined 68 sera of patients who were referred to our department with suspected Borreliosis or Tick-Borne Encephalitis (TBE). The detection of antibodies against E. chaffeensis was performed by indirect immunofluorescence assay. Five (45.4%) patients of 11 were diagnosed as Lyme borreliosis confirmed by serum presence of antibodies against B. burgdorferi. Three patients (27.2%) showed serum presence of antibodies against TBE. In one case presence of antibodies against three tick-borne agents was stated. None of our patients presented leucopenia characteristic of ehrlichiosis. trombocytopenia or increased aminotransferase level.