The seroprevalence of human granulocytic anaplasmosis (former human granulocyticehrlichiosis, HGE) has been documented in several studies, but little data exists on incidence ratesin healthy individuals. In a prospective study, we tested 125 healthy adults (mean age 43 years)--workersof the Bialowieza Primeval Forest National Park, north-eastern Poland--for Anaplasma phagocytophilumIgG antibodies using an indirect immunofluorescence antibody assay, and for Borrelia burgdorferi IgGwith ELISA in a 12-month interval. The data concerning clinical symptoms consistent with human granulocyticanaplasmosis were collected using a standardized questionnaire. Of these 125 subjects, 9 were anti-A.phagocytophilum positive at the study entry. Four participants (3.2 %) seroconverted from IgG negativeto positive during the observation period. Three subjects (2.4 %) converted from initially anti-A. phagocytophilumpositive to negative. Specific IgG antibodies against Borrelia burgdorferi were detected in 27 (21.6%) individuals. Concurrence of Borrelia burgdorferi and Anaplasma phagocytophilum was observed in 3.2%, whereas 4 % were Anaplasma phagocytophilum IgG positive and Borrelia burgdorferi IgG negative (notsignificant). Clinical symptoms associated with human granulocytic anaplasmosis were not present in seroconvertingindividuals. The obtained results confirm the occurrence of Anaplasma phagocytophilum infection in north-easternPoland with asymptomatic clinical course.