Ehrlichia are obligate intracellular bacteria that belong to the family Rickettsiaceae. Human monocytic ehrlichiosis (HME) and human granulocytic ehrlichiosis (HGE) are the 2 ehrlichial diseases that are of greatest health concern in the United States. The agents causing HME and HGE are zoonotic pathogens requiring a mammalian reservoir and an arthropod vector. Differences in the geographic distribution of their tick vectors account for the concentration of HME in the South and southeastern United States and HGE in the Northeast and northern Midwest. Both infections have been reported in coastal regions from Rhode Island to Florida. HME and HGE are flulike illnesses that usually are self-limited but may be fatal. Diagnosis is confirmed by identification of the microorganism on blood smear or polymerase chain reaction or by detection of anti-ehrlichial antibody. Doxycycline is the antibiotic of choice for treatment of ehrlichiosis.