OBJECTIVE: To test the performances of new Borrelia garinii immunoblots specific for Borrelia burgdorferi sensu lato with a selected panel of sera from patients with various clinical presentations of Lyme borreliosis. METHODS: In order to establish the sensitivity and the specificity of these immunoblots, we tested serum samples obtained from patients with early- and late-stage Lyme disease (erythema migrans n=35, neuroborreliosis n=61, acrodermatitis chronica atrophicans (ACA) n=27 and arthritis n=41), from patients with diagnoses and laboratory findings associated with serologic cross-reactivity to Lyme disease (syphilis n=12, Epstein-Barr infection n=9, autoimmune markers n=29) and from blood donors residing in regions of low and medium endemicity (n=80, n=100). RESULTS: The combined sensitivity (IgG and IgM) of the tests was 90% for patients with erythema migrans, 92% for neuroborreliosis, 96% for ACA and 100% for Lyme arthritis. The specificity of the IgG immunoblot was 94%, and that of the IgM immunoblot was 97%, taking into account the prevalence of borrelia antibodies in the overall population. Interpretation of these immunoblots is based on scores allocated to different specific borrelia antigens. CONCLUSIONS: The Western blot technology is extremely useful in dissecting the immune response to borrelia infections, which develops gradually over a period of weeks to years and which involves the appearance of IgM and IgG antibodies directed against a number of borrelia-associated proteins.