In Europe and in other parts of the world, Lyme borreliosis is recognized increasingly by physicians and serodiagnostic laboratories. However, it is currently difficult to present conclusive epidemiologic data. There are no widely accepted case definitions for the numerous clinical features due to or supposed to be linked to infection with Borrelia burgdorferi. Clinical diagnosis of suspected cases of Lyme borreliosis requires confirmation by the demonstration of the aetiologic agent and the recognition of its causative role in the respective disorder. The specificity of serological tests for Lyme borreliosis is impaired by several phenomena including cross-reacting antibodies. Interpretation of serological test results may lead to the clinical diagnosis of Lyme borreliosis and in consequence to antibiotic treatment. Complications in order to treat suspected disseminated Lyme disease may be severe, as recently reported (45). No reliable data of Lyme borreliosis prevalence can be offered so far. This is partly due to incomplete reporting of Lyme borreliosis cases to specialized institutions, and partly due to the insufficiency of the diagnostic process. The true incidence and prevalence of this disease cannot be determined and one must wait for the results of the development of specific and dependable methods to identify actual infection.