We recently reported a novel Ehrlichia sp. closely related to E. muris detected in blood of 4 from patients in Minnesota (MN) and Wisconsin (WI) in 2009 (NEJM 2011). We now present data from the 2009-2011 patient cases, and results of tick, rodent, and deer studies. Blood from patients with suspected ehrlichiosis or anaplasmosis was tested using PCR targeting the Ehrlichia/Anaplasma Heat Shock Operon gene. PCR was also performed on rodent and deer blood, ticks from MN and WI, and ticks found on military personnel at 132 U.S. military bases nationwide (2007-2010). Select specimens were characterized using culture and DNA sequencing. Human sera were tested for antibodies to specific Ehrlichia spp. and A. phagocytophilum. During 6/2009-8/2011, blood from 32 of 8,110 MN and WI patients tested PCR positive for an Ehrlichia sp. other than E. chaffeensis and E. ewingii. This result was also detected from 1 North Dakota patient who recently travelled to MN. The result was not noted among 7,827 other patient specimens from other states. An Ehrlichia sp. was also cultured from the blood of one WI patient. The 16S rRNA gene (rrs) sequence of clinical and culture isolates was most similar to E. muris (98%). Patients comprised 22 men and 11 women, aged 23 - 87 years. Among patients with available data, infection manifested with fever (31/33), headache (26/33) lymphopenia (7/12) and thrombocytopenia (14/19); 4 of 33 were hospitalized (≤3 days) and 32 of 33 recovered following doxycycline treatment. One patient recovered without treatment. Three of 3 patients had a higher antibody titer to the novel Ehrlichia sp. than E. chaffeensis. No antibodies to A. phagocytophilum antigens were detected. Thirty-four of 1,384 I. scapularis ticks from MN and WI were PCR positive for the novel Ehrlichia, whereas it was not detected in I. scapularis from other states (n=2931) or other tick species (n=6563). Two of 147 rodents and 0 of 180 deer tested were PCR positive. Based on these data, the novel Ehrlichia sp. appears to circulate in a region where Ehrlichiae have not historically been considered endemic and causes a disease which is clinically and epidemiologically similar to human monocytic ehrlichiosis due to E. chaffeensis. These findings should be taken into consideration for diagnosis and surveillance.