Lyme disease constitutes a major health hazard with an increased incidence throughout the United States, in particular the eastern states. Human ehrlichiosis, also a tick-borne illness, has recently been identified. It is characterized by fever, headache, malaise, leukopenia, thrombocytopenia, and elevated liver enzyme titers, and has been reported to occur mainly in the South Central and South Atlantic states. As with Lyme disease, most patients have a history of tick exposure. These two diseases may be difficult to differentiate clinically. Physicians must consider the possibility of both infections when patients become ill with a systemic illness after tick exposure. Although certain demographic and clinical features are characteristic of these diseases, they can be misleading. Only serologic evidence can confirm the diagnosis. Two cases of concurrent Borrelia and Ehrlichia infections have been previously reported. The authors herein describe a third case that further illustrates the potential diagnostic dilemma posed by the concurrence of these two entities.