Objective. - To characterize the clinical and laboratory features observed in patients with human granulocytic ehrlichiosis (HGE) and evaluate the utility of the diagnostic tools used to confirm the diagnosis. Design. - Retrospective case study of 41 patients with laboratory-diagnosed HGE. Setting. - A total of 228 patients from Minnesota and Wisconsin were evaluated between June 1990 and May 1995. Methods. - Cases were presumptively identified by a history of an influenzalike illness acquired in an area known to be endemic for ticks. Diagnostic laboratory testing included microscopic examination of Wright-stained peripheral blood smears for presence of neutrophilic morulae, polymerase chain reaction (PCR) analysis of acute- phase blood samples for the Ehrlichia phagocytophila/Ehrlichia equi group DNA, and evaluation of serological responses by indirect immunofluorescent antibody assay (IFA), using E equi as antigen. Results. - All patients presented with a temperature of at least 37.6°C, and most had headache, myalgias, chills, and varying combinations of leukopenia, anemia, and thrombocytopenia. Eighty percent of the patients tested demonstrated morulae in the cytoplasm of peripheral blood neutrophils. Only 16 of 37 patients tested by PCR were positive for HGE, whereas serum IFA assays of acute or convalescent blood samples detected antibodies against E equi in 38 of 40 patients tested. Two patients died, and the calculated case fatality rate was 4.9%. Conclusions. - Human granulocytic ehrlichiosis is being increasingly recognized in Wisconsin and Minnesota. A more severe illness is associated with increased age, anemia, increased percentage of neutrophils and decreased percentage of lymphocytes in peripheral blood, and presence of morulae in neutrophils. The differential diagnosis for patients who develop an influenzalike illness following a tick bite should include HGE. Microscopic examination of the acute-phase blood smear to detect neutrophilic morulae is currently the quickest and most practical screening method for diagnosing HGE in the upper Midwest.