Abstract: In early September 2010,the laboratory detection and epidemiological survey of 4 suspected cases of human granulocytic anaplasmosis(HGA) conducted by Shandong Provincial Center for Disease Control and Prevention indicated that the 4 cases were caused by co-infection of Anaplasma phagocytophilum and Ehrlichia chaffeensis, which met the clinical diagnostic criteria set in the Guideline of prevention and control on human granulocytic anaplasmosis (for trial) issued by the Ministry of Health.