The ehrlichioses are emerging zoonotic infections that are caused by obligate intracellular bacteria in the genus Ehrlichia. Two human ehrlichioses occur in the United States: monocytic ehrlichiosis (HME), which is caused by Ehrlichia chaffeensis that infects mononuclear phagocytes in blood and tissues, and granulocytic ehrlichiosis (HGE), an infection of granulocytes that is due to a phylogenetically distinct organism. Both infections cause undifferentiated fever with leukopenia, thrombocytopenia, and elevations in serum aminotransferase levels. Rash is an infrequent sign, and vasculitis is exceedingly rare. Severe or fatal ehrlichiosis is associated with secondary or opportunistic infections and delayed therapy. Ticks are the likely vectors, and deer are the likely reservoirs. HGE is associated with Ixodes species ticks and Lyme disease, a finding suggesting concurrent infection. In cases of HME, ehrlichial inclusions (morulae) are rarely detected; however, they are often seen in neutrophils of patients with HGE. A clinical diagnosis is confirmed with use of the polymerase chain reaction during the infection or by serology during convalescence. Therapy with doxycycline is highly efficacious.