This paper reports a 17-year seroepidemiological surveillance study of Borrelia burgdorferi infection, performed with the aim of improving our knowledge of the epidemiology of this pathogen. Serum samples (1,179) from patients (623, stratified with respect to age, sex, season, area of residence and occupation) bitten by ticks in two regions of northern Spain were IFA-tested for B. burgdorferi antibodies. Positive results were confirmed by western blotting. Antibodies specific for B. burgdorferi were found in 13.3% of the patients; 7.8% were IgM positive, 9.6% were IgG positive, and 4.33% were both IgM and IgG positive. Five species of ticks were identified in the seropositive patients: Dermacentor marginatus (41.17% of such patients) Dermacentor reticulatus (11.76%), Rhiphicephalus sanguineus (17.64%), Rhiphicephalus turanicus (5.88%) and Ixodes ricinus (23.52%). B. burgdorferi DNA was sought by PCR in ticks when available. One tick, a D. reticulatus male, was found carrying the pathogen. The seroprevalence found was similar to the previously demonstrated in similar studies in Spain and other European countries.