Lyme borreliosis is a tick-borne disease caused by the bacterium Borrelia burgdorferi. The most frequent clinical manifestation is a rash called erythema migrans. Changes in antibody reactivity to B. Burgdorferi 3 months after a tick bite are measured using enzyme-linked immunosorbent assays (ELISAs). One assay is based on native purified flagellum antigen (IgG), and the other assay is based on a recombinant antigen called C6 (IgG or IgM). Paired samples were taken at the time of a tick bite and 3 months later from 1,886 persons in Sweden and the Aland Islands, Finland. The seroconversion or relative change is defined by dividing the measurement units from the second sample by those from the first sample. The threshold for the minimum level of significant change was defined at the 2.5% level to represent the random error level. The thresholds were a 2.7-fold rise for the flagellar IgG assay and a 1.8-fold rise for the C6 assay. Of 1,886 persons, 102/101 (5.4%) had a significant rise in antibody reactivity in the flagellar assay or the C6 assay. Among 40 cases with a diagnosis of Lyme borreliosis, the sensitivities corresponding to a rise in antibodies were 33% and 50% for the flagellar antigen and the C6 antigen, respectively. Graphical methods to display the antibody response and to choose thresholds for a rise in relative antibody reactivity are shown and discussed. In conclusion, 5.4% of people with tick bites showed a rise in Borrelia-specific antibodies above the 2.5% threshold in either ELISA but only 40 (2.1%) developed clinical Lyme borreliosis.