Ehrlichiosis is the potentially life-threating infection. It is caused by obligate intracellular bacteria. The clinical presentations are fever, headache, myalgia, malaise, nausea, vomiting and other nonspecyfic symptoms. Some patients develop neurologic symptoms and signs. The are two distinct forms of human ehrlichiosis: human monocytic ehrlichiosis /HME/--cased by Ehrlichia chaffeensis that infects mononuclear phagocytes and human granulocytic ehrlichiosis /HGE/--caused by E. species closely related to E. phagocytophyla and E. equi and infects granulocytes. Successful treatment of these infections may depend on proper diagnosis. Appropiate diagnostic tests are still not available. This diagnisis should be considered in febrile patients with tick bites.