The aim of this study was to acquire a better knowledge of the epidemiology of Lyme disease. A seroepidemiologic study was made by evaluating the different levels of antibodies to Borrelia burgdorferi in a five hundred serum representative sample of a population from the province of La Rioja, Spain. Samples were randomly selected and stratified to take into account sex, age, rural or urban residence, and geographical area. All subjects included in the study were asked to fill out a questionnaire containing this information along with data concerning leisure activities, occupation, contact with animals, tick-bite and antecedent features related to Lyme disease. No statistical differences were found in relation to sex or age. The seroprevalence was very high (31.3%) in mountain areas, where Ixodes ricinus and deer are present. The risk factors associated with seropositivity were forestry and cattle-raising activities, as well as contact with animals. The best titer of antibodies associated with clinical antecedents of Lyme disease and related disorders was 1/256.