Ehrlichiae are tick-borne rickettsial organisms that are well known as veterinary pathogens. However, since 1986, over 100 cases of human infection by Ehrlichia canis or a closely related species have been identified primarily in the southeastern and south central United States. Human ehrlichiosis is characterized by high fever, rigors, headache, myalgia, anorexia, and, sometimes, gastrointestinal complaints. Rash occurs in a minority of cases. Commonly observed laboratory abnormalities include leukopenia, thrombocytopenia, anemia, and elevated hepatic aminotransferase levels. The illness is tick-borne and tetracycline or tetracycline analogs appear to be effective in treating the illness.