The seroprevalence of granulocytic ehrlichiosis has been documented in several studies, but little data exists on incidence rates. Using sera stored from an earlier study on Lyme borreliosis, 290 residents of Aspö Island could be followed prospectively during two tick seasons (1992-1994). Immunoglobulin G antibodies to granulocytic ehrlichiosis were detected by an immunofluorescence assay using Ehrli- chia equi as antigen. Seroprevalence rates increased significantly over time, and at the 1994 follow-up, 28% of the residents were seropositive. Negative-to-positive seroconversion (incidence) rates were 3.9% and 11.1%, respectively, during the two seasons. A highly significant correlation was found between a positive serologic response for granulocytic ehrlichiosis and Borrelia burgdorferi. No such correlations were found for clinical Lyme borreliosis, self-reported arthralgia or number of recorded tick bites. It was concluded that granulocytic ehrlichiosis is highly endemic in this part of Sweden, with a seroconversion rate as high as 11% over a single tick season. Further studies are necessary to correlate these findings with clinical signs of human granulocytic ehrlichiosis.