Since 1986, two human tickborne diseases caused by Ehrlichia spp. have been recognized in the United States: human monocytic ehrlichiosis (HME), caused by E. chaffeensis, and human granulocytic ehrlichiosis (HGE), caused by an agent closely related to E. equi (1,2). In June 1995, the Westchester County (New York) Department of Health (WCDOH) received reports from physicians who were treating patients for suspected HGE. In response, the WCDOH sent information to all primary-care physicians in Westchester County describing the clinical and laboratory features of ehrlichiosis (fever, myalgia, headache, leukopenia, and thrombocytopenia) and requested that they voluntarily report suspected cases of ehrlichiosis. This report summarizes an investigation by the New York State Department of Health (NYSDOH) and the WCDOH of suspected ehrlichiosis cases and the clinical characteristics of confirmed and probable cases.