Human granulocytic ehrlichiosis is a potentially fatal tick-borne infection that has recently been described. This acute febrile illness is characterized by myalgias, headache, thrombocytopenia, and elevated serum aminotransferase levels. The disease is difficult to diagnose because the symptoms are non-specific, intraleukocytic inclusions (morulae) may not be seen, and the serologic results are often initially negative. Little is known about the causative agent because it has never been cultivated.