Forty-six patients with late Lyme disease who were considered improved or cured following treatment were monitored by immunoglobin M (IgM) immunoblotting (mean monitoring period, 27.6 months). There was a persistent IgM response in 32 (97%) of 33 initially positive patients. All but three showed a consistent number, type, and intensity of IgM bands over the entire follow-up period. IgM immunoblotting may not be useful for monitoring the response to treatment of Lyme borreliosis.