Erythema chronicum migrans (ECM), lymphadenosis cutis benigna and acrodermatitis chronica atrophicans (ACA) have been well known in Europe for a long time as skin diseases induced by tickbites. Earlier hints that these inflammatory dermatoses and associated disorders (erythema migrans disease) might be of spirochetal origin are now supported by the findings of several groups in the USA and Europe. In the United States the endemic occurrence of a new inflammatory disease which seems to be closely related to the European erythema migrans disease was first seen in Lyme, Connecticut, in 1975. This "Lyme disease" is also induced by tick bites and affects mainly skin and joints. In 1982 the isolation of a new spirochetal species cultivated from Ixodes dammini ticks was reported. Antibodies against these Ixodes dammini spirochetes (IDS) were detected by indirect immunofluorescence in patients with Lyme disease. In 1983 spirochetes were shown by Warthin-Starry silver stain in skin biopsies of erythema chronicum migrans, and nearly simultaneously the isolation of spirochetes similar to the IDS from skin, blood and cerebrospinal fluid of Lyme patients was reported in a few cases. Also, in Europe Borrelia-like spirochetes morphologically very similar to the IDS have been isolated from ticks of the indigenous species Ixodes ricinus by transmission to laboratory animals and cultivation in vitro. Antibodies against Ixodes ricinus spirochetes (IRS), but also against known Borrelia species have been shown in most sera of patients with erythema migrans disease, including meningoradiculitis Bannwarth, acrodermatitis chronic atrophicans and lymph-adenosis cutis benigna.(ABSTRACT TRUNCATED AT 250 WORDS)