Human granulocytic ehrlichiosis (HGE) is a tickborne illness caused by an agent closely related to Ehrlichia equi and Ehrlichia phagocytophila. The clinical presentation is nonspecific, and diagnosis is made infrequently. Sixty-six sera were obtained from 54 patients in Minnesota and Wisconsin with undifferentiated fever during the summer and fall of 1993. Serologic reactivity with E. equi was observed in 6 (11%), including 4 seroconversions, 1 stable titer, and 1 seroreversion. Of the seropositive patients, HGE agent DNA was detected by polymerase chain reaction in the first serum from 4 of 5 patients and was present in the serum of 1 of 2 untreated patients after 21 days, even when E. equi antibodies also were present. HGE is a significant and potentially frequent cause of undifferentiated fever in Wisconsin and Minnesota during seasons with tick activity. The agent may persist in untreated patients for at least 1 months or may be cleared earlier, even if not treated with doxycycline or tetracycline.