We conducted a seroprevalence survey of antibodies to Borrelia burgdorferi in West Central Wisconsin, an area endemic for Lyme disease. One hundred and thirty-seven of 153 patients having blood drawn at the Eau Claire Family Medicine Clinic and the associated rural Augusta practice participated in the study and were tested for the presence of antibodies to B burgdorferi and questioned about Lyme disease risk factors. An enzyme-linked immunosorbent assay showed that 15 (10.9%) of the 137 persons had serological evidence of exposure to B burgdorferi. None of these 15 persons reported a past or current history of clinical Lyme disease, and they were not found to have more symptoms commonly associated with Lyme disease than seronegative patients. We conclude that the prevalence of positive Lyme serologies in this area is similar to that of other endemic areas and that subclinical or asymptomatic infection probably accounts for many of these positive results.