Lyme spirochaetal disease (LSD) is a complex multisystem disorder which has been recognized as a separate entity due to its close geographic clustering of affected patients. The study aimed at evaluating the clinical and immunological features of LSD with chronic symptoms of meningoradiculitis, carditis and pauciarticular arthritis. Six patients with LSD and erosive arthritis who developed an increase of serum IgM rheumatoid factor (RF) which correlated with the inflammatory activity of the disease are described in detail. Besides raised IgG antibody titers to Borrelia burgdorferi (B. burgd.) antigen measured by ELISA technique, circulating immune complexes, antinuclear antibodies (ANA) and RF measured by laser nephelometric immunoassay were detected. Increased ANA and RF antibody rates suggest that LSD may closely be linked with transient autoimmune phenomena. Thus, in some cases, B. burgd. antigens might be able to produce a strong polyclonal B-cell stimulation, hence leading to an unspecific autoimmune reaction. But the question remains if transient unspecific autoimmune reactions actually take part in the pathogenesis of LSD.