Forms of lymphadenosis benigna cutis (Lbc) which run an unusual course may present considerable problems and are sometimes difficult to distinguish from malignant lymphomas of the skin on the basis of the clinical and histological picture. The case of a female patient is reported, who presented with plaque-like infiltrative lesions on both lower arms. The lesions were interpreted as being malignant lymphomas of the skin. After a progressive course of 6 years' duration, the diagnosis of an unusual plaque-like infiltrative form of Lbc caused by Borrelia was serologically confirmed by the demonstration of anti-Borrelia burgdorferi antibodies. Immunohistochemical examinations yielded the picture of a follicular pseudolymphoma with the formation of germ centres. After several weeks of penicillin therapy the skin lesions cleared completely.