The aim of this study was to report the clinical, serological and epidemiological features of a homogeneous cohort of patients with various forms of Lyme borreliosis, based on recent European case-definition guidelines. Complete clinical and epidemiological reports were requested from the patients' physicians for case definition. Enzyme immunoassay IgG and IgM screening tests were performed. A total of 170 patients (141 adults and 29 children) with the following forms of Lyme borreliosis were included between 1989 and 1997: erythema migrans alone (n = 38), early neuroborreliosis (n = 76), arthritis (n = 34), acrodermatitis chronica atrophicans (n = 12), carditis (n = 2), lymphocytoma (n = 3), chronic neuroborreliosis (n = 2), and miscellaneous disorders (n = 3). On the basis of the occurrence and course of the full spectrum of complicated forms of Lyme borreliosis, the specificity of the clinical and biological presentation of the complicated forms (age, ratio of children/adults, sex ratio, incubation period, time to diagnosis, and serological profiles) was demonstrated, as was the absence of overlap between the clinical presentations. Using these data, an alternative scheme for the natural history of Lyme borreliosis is suggested.