Spirochetes were recovered from the skin lesion of 1 out of 10 acrodermatitis chronica atrophicans patients (ACA). Spirochetes from this skin isolate and from Ixodes (I.) ricinus and I. dammini spirochetes were used as antigens in indirect immunofluorescence tests. All sera from 17 ACA patients showed high antibody titers to the three antigens. Seven of the 17 sera which had the highest titers had crossreactive antibodies to treponemal antigen detectable in the FTA-ABS test. The results indicate that spirochetes are of importance for ACA and probably the causative agent of this disease. The connection between ACA and tick bites and the relationship to erythema chronicum migrans Afzelius (ECMA) and Lyme disease are discussed. The results are consistent with the hypothesis that ECMA and ACA are different manifestations of the same spirochete, with ACA as a late manifestation.