More than 20 years after the first description of the pathogen Borrelia burgdorferi the diagnosis of Lyme borreliosis is based primarily on clinical criteria. Multiple differential diagnoses have to be considered and laboratory confirmation based on various test procedures is required until now. For screening assays the immunofluorescence assay (IFA), enzyme linked immunoassay (EIA), or indirect hemagglutination assay (IHA) are frequently in use. During the last ten years a stepwise diagnostic procedure in USA and Europe has been recommended. The use of a serological screening assay as the first diagnostic step, followed by a confirmatory assay only in the case of a positive or borderline result of the screening assay is now "state of the art". The most frequently used method is a sensitive enzyme linked immunoassay followed by a sensitive and specific immunoblot technique. However, the quality of these indirect serological tests and, mbst importantly, the interpretation criteria of the commercial tests vary dramatically. The choice of Borrelia strains, antigen preparation, production conditions and test procedures vary widely. The immunoblot, used as a confirmatory assay, must meet the highest quality standards but in practice under routine laboratory conditions, these levels are often not reached. The major reason for this "shortcoming" is that Lyme borreliosis is not a sexually transmitted disease and no official approval has been required by the commercial suppliers so far. Another increasingly important aspect is the introduction of molecular biological assays into laboratory routines. Numerous authors published many articles about the usefulness of polymerase chain reaction as a diagnostic tool for the detection of borreliae in human specimens. However, a routine method has not yet been established. Furthermore, no standardized method for DNA isolation from different human specimens has been developed so far. For this reason PCR is not accepted as a routine method and the application should be restricted to a few laboratories with experience in the field of Lyme borreliosis. In summary, it is important to inform clinicians about the limitations of the tests used and it would be very helpful to stage a European or worldwide conference to establish accepted rules for the diagnosis of Lyme borreliosis.