Human granulocytic anaplasmosis, formerly known as human granulocytic ehrlichiosis, is caused by the microorganismAnaplasma phagocytophilumthat is transmitted byIxodestick bites. The disease state ranges from subclinical to fatal but may be difficult to differentiate from other febrile conditions without specific tests. Rapid and early diagnosis is important since the infection may be fatal and specific antibiotic therapy is required. The bacterium is an obligate intracellular pathogen of neutrophils. Thus, early diagnosis is best achieved by amplification of nucleic acids from the blood. An increasing number of potential gene targets for diagnostic assays have been described and the incipient release of anAnaplasma phagocytophilumgenome sequence will not only help to better understand the disease but may facilitate improvements in diagnostic strategies.