Ten patients with a clinical diagnosis of lymphadenosis benigna cutis (LABC) solitaria were investigated. Four of the patients knew that they had received a tick bite during the year prior to the appearance of the LABC lesion. Five patients had had a migrating erythema, indicating a previous erythema chronicum migrans Afzelius, near the site where the LABC lesion had developed. In one patient meningitis and a facial palsy occurred. Elevated titers of antibodies against Borrelia spirochetes were found in sera from six of nine patients. Spirochetes were cultivated from one out of two biopsies from LABC lesions. The clinical findings, the results of the serologic tests and the successful cultivation of spirochetes show that Ixodes ricinus-transmitted Borrelia spirochetes can cause LABC solitaria.