From three endemic locations of erythema chronicum migrans disease in North Rhine-Westphalia, Germany, we recovered 19 isolates of a spirochete from Ixodes ricinus ticks. The infection rate in adult ticks was 16 percent. The isolated spirochete is immunologically related to the Ixodes dammini spirochete, Borrelia duttoni, and Treponema pallidum. Using indirect immunofluorescence, the sera of 90 patients with erythema chronicum migrans disease showed antibody titers against the isolated spirochete, which correlated with the clinical course. Similarly, antibodies were demonstrated in the sera of 21 patients with acrodermatitis chronica atrophicans. These results suggest an etiologic role for the Ixodes ricinus spriochete in European erythema chronicum migrans disease
