An evaluation of the clinical outcome and the duration of the antibody response of patients with human granulocytic ehrlichiosis (HGE) was undertaken in Slovenia. Adult patients with a febrile illness occurring within 6 weeks of a tick bite were classified as having probable or confirmed HGE based on the outcome of serological or PCR testing. Thirty patients (median age, 44 years) were enrolled, and clinical evaluations and serum collection were undertaken at initial presentation and at 14 days, 6 to 8 weeks, and 3 to 4, 6, 12, 18, and 24 months. An indirect immunofluorescence assay (IFA) was performed, and reciprocal titers of > or =128 were interpreted as positive. Patients presented a median of 4 days after the onset of fever and were febrile for a median of 7.5 days; four (13.3%) received doxycycline. Seroconversion was observed in 3 of 30 (10.0%) patients, and 25 (83.3%) showed >4-fold change in antibody titer. PCR results were positive in 2 of 3 (66.7%) seronegative patients but in none of 27 seropositive patients at the first presentation. IFA antibody titers of > or =128 were found in 14 of 29 (48.3%), 17 of 30 (56.7%), 13 of 30 (43.4%), and 12 of 30 (40.0%) patients 6, 12, 18, and 24 months after presentation, respectively. Patients reporting additional tick bites during the study had significantly higher antibody titers at most time points during follow-up. No long-term clinical consequences were found during follow-up.