In order to evaluate the virtual advantage of proving borrelial infection in dermatoses clinically diagnosed as erythema (chronicum) migrans (ECM), acrodermatitis chronica atrophicans (ACA) or lymphadenosis cutis benigna (LCB) by means of histological, cultural or serological trials, skin and serum samples obtained from altogether 99 patients suffering from these dermatoses were examined. Serum and--to some extent--skin specimens gained from healthy individuals (n = 36), patients with not tick bite associated disorders of skin, internal organs and nervous system (n = 121) were used as controls. In addition, serum specimens from patients enduring circumscribed scleroderma (morphea), n = 31, and sera of Bavarian forest workers (n = 211) were proven for the presence of borrelial antibodies. Using an indirect immunofluorescence assay with borrelial strains, propagated in inbred mice, as antigens, elevated IgM- and/or IgG antibody titers were found in 51 (84%) out of 61 ECM-, in 17 (100%) ACA- and in 2 (33%) out of 6 LCB-sera. Antibody titers decreased significantly after adequate antibiotic therapy. The groups in comparison yielded spirochetal serum antibodies as follows: Healthy individuals and patients with various diseases 7 out of 157 (4.5%), morphea patients 7 out of 31 (23%) and forest workers 71 out of 211 (34%). Borrelia burgdorferi was cultivated from 6 out of 32 skin and from 1 blood specimen obtained from ECM patients and from 1 out of 5 ACA skin specimens. Borreliae could also be detected in the blood of 3 out of 11 thymusaplastic nude mice after intraperitoneal implantation of ECM-tissue. More frequently than borreliae spindle-shaped bacteria resembling fusobacterium sp. were detected not only by in vitro cultivation of ECM and ACA skin samples, but also in the blood of thymusaplastic mice after implantation of ECM-tissue. By Warthin-Starry silver strain borrelia-like structures were detected in 9 out of 28 ECM, in 3 out of 12 ACA and in 1 out of 5 LCB skin specimens. Therefore in our experience serological examinations turned out to be the most productive diagnostic tool.