The purpose of this study was to examine the seasonal variations in seroprevalence to Ehrlichia phagocytophila in cattle pastured during the summer months in an area where tick-borne fever is endemic. The study was performed during a 1-year period from April 1996 to March 1997 and involved 34 cows, 22 pregnant heifers, and 14 calves. Blood samples, collected from all 70 cattle once a month, were used to determine serum immunoglobulin G titers by indirect immunofluorescence. In addition, blood smears were examined for Ehrlichia organisms, and PCR amplification was performed for the molecular detection of E. phagocytophila. Prior to the pasture period, the seroprevalence was 16%. Two weeks after the start of pasturing, it was 43%, after which it progressively increased and reached a maximum of 63% in September. The seroprevalence progressively decreased after the end of pasturing to a low of 23%. The variation in antibody titers was similar to that of seroprevalence. E. phagocytophila organisms were detected in blood smears of 7 animals and by nested PCR in 12. Only four cows, which were on the pastures of endemicity for the first time, had clinical signs of ehrlichiosis. This study demonstrated marked seasonal variations in seroprevalence and in serum titers of antibody to E. phagocytophila in cattle. The incidence of clinical signs of ehrlichiosis was increased in cattle grazing on the pastures of endemicity for the first time.