In Lyme's disease (LD) as well as in the European form of erythema chronicum migrans (ECM), the etiologic agents are spirochetes. As fas as we know by now, these microbes are closely related but not identical. Consequently, LD and ECM should be regarded as closely related but not as identical diseases. The sera of our 21 patients suffering from acrodermatitis chronica atrophicans (ACA) contained elevated antibody titers directed against the etiological agent found in ECM. These findings strongly suggest that ACA is also induced by spirochetes-possibly by the same microbes found in ECM.