Human ehrlichiosis was first described in the United States in 1986. Since then, more than 215 cases have been reported, including some fatalities. Ehrlichia species belong to the same family as the organism that causes Rocky Mountain spotted fever. Human ehrlichiosis occurs most frequently in the southern mid-Atlantic and south-central states, during spring and summer months. The clinical presentation is similar to that seen in Rocky Mountain spotted fever although, with ehrlichiosis, leukopenia is more often found and skin rash is less often noted. Definitive diagnosis is based on acute and convalescent serum antibody titers. Ehrlichiosis cannot reliably be distinguished from other common febrile illnesses on the basis of clinical, epidemiologic or laboratory features. Therapy must be initiated empirically in suspected cases. Both ehrlichiosis and Rocky Mountain spotted fever respond well to tetracycline and chloramphenicol, but not to penicillins or cephalosporins.