A study was made of 50 consecutive patients with untreated acrodermatitis chronica atrophicans (ACA). In all patients elevated anti-spirochetal antibody titers were found at indirect immunofluorescence and enzyme-linked immunosorbent assays, and histologically biopsies exhibited a dermal lymphocytic infiltrate with a moderate to rich admixture of plasma cells and telangiectases. Nine patients had a history consistent with spontaneously healing erythema chronicum migrans Afzelius (ECMA) on the extremity on which, after 0.5-8 years, ACA lesions developed. Eight patients had a history indicating previous cranial nerve involvement and nine had had periods of severe pains in the cervical or lumbar region. Two patients had developed ECMA, facial palsy and ACA in chronological order. In 15 patients radiographic abnormalities of joints and/or bone tissue were found. Besides ACA lesions, lichen sclerosus et atrophicus- or scleroderma-like lesions were found in six patients. The inflammatory ACA lesions were sometimes discrete and had been overlooked. Joint deformities, sclerotic lesions, diffuse edema or pain were the cardinal symptoms in some patients. The findings indicate that clinical recognition of ACA may be difficult and that a combination of clinical, histopathologic and serologic findings may be necessary to establish the diagnosis. The results are consistent with the concept that ACA is a late manifestation of infection by the same spirochete as causes ECMA and Bannwarth's syndrome.