We investigated the appearance and evolution of immunoglobulin M (IgM) and IgG antibodies to Borrelia burgdorferi in 46 patients with culture-proven erythema migrans (EM). All patients received antimicrobial treatment and were prospectively evaluated for up to 1 year. A total of 257 serially collected serum samples were tested by commercial IgG-IgM enzyme-linked immunosorbent assay and separate IgM and IgG immunoblots (IBs). At the baseline, 33% of the patients had a positive ELISA result and 43% of the patients had a positive IgM IB result by using the criteria of the Centers for Disease Control and Prevention-Association of State and Territorial Public Health Laboratory Directors for the interpretation of IB results. Positive serology at the baseline and the rate of seroconversion correlated directly with disease duration and/or evidence of dissemination prior to treatment. At days 8 to 14 after the baseline, 91% of patients had a positive ELISA result and/or IgM IB result. Peak IgM antibody levels were seen at this time in patients with localized or disseminated disease. The most frequent IgM bands at the baseline and the peak were of 24 kDa (OspC), 41 kDa, and 37 kDa. Although 89% of the patients developed IgG antibodies as determined at a follow-up examination, only 22% were positive by the IgG IB criteria of the Centers for Disease Control and Prevention-Association of State and Territorial Public Health Laboratory Directors. The persistence of antibodies was directly related to disease duration and/or dissemination prior to treatment. Since IgM antibodies to the 24- and 41-kDa antigens remained detectable for long periods, 38% of IgM IBs were still positive at 1 year postbaseline. IgM to antigens of 39, 58, 60, 66, or 93 kDa, conversely, were most often seen in sera obtained within 1 month postbaseline. Their presence may be of assistance in confirming a recent infection with B. burgdorferi in individuals living in areas where Lyme disease is endemic.