As long as test procedures are not standardized, the serological results of IgM- and IgG-antibodies in Lyme borreliosis must be interpreted with caution and always in the context of clinical signs and symptoms. False negative results occur primarily during the first weeks of infection. In erythema migrans of less than 4 weeks' duration, 50% of patients are seronegative even with newly designed ELISAs. At this early stage of the infection the therapeutic decision has to be established on the basis of clinical criteria. Frequently IgM- and/or IgG-antibodies develop during antibiotic therapy. After 4 weeks' duration 80% of patients have elevated borrelial antibodies detectable with recently developed ELISAs. Positive and borderline results should be confirmed by Western blot. False positive results, particularly slightly elevated IgM, may occur in a variety of other diseases. Another problem is the persistence of Borrelia-specific IgM antibodies after therapy. Serological follow-up can only be carried out with the same methods in the same laboratory. Retreatment should be considered if IgM antibodies are increasing significantly and new symptoms are occurring.