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Four patients with low-grade malignant B cell lymphoma of the skin in association with chronic Borrelia burgdorferi infection are presented. Plaque-shaped or nodular erythematous lesions with ill-defined borders were seen. Clinical progression was slow; the skin tumors occurred for up to 7 to 15 years. Extracutaneous involvement was found in only one case. Immunohistologic investigations showed an expression of B cell markers with restriction to only one light chain type and absence of T cell antigens. The growth fraction was 5% to 30%, as shown with the monoclonal antibody Ki-67. The immunoarchitecture of the tumors in three patients was unusual compared with established criteria for cutaneous B cell lymphoma and revealed similarities to mucosa-associated B cell lymphoma. Some immunohistologic heterogeneity may indicate the development of monoclonal proliferation that originated from different phases of B lymphocytic differentiation. In three cases no clinical signs of B. burgdorferi infection were found; in one patient acrodermatitis chronica atrophicans was present. The occurrence of acrodermatitis chronica atrophicans and malignant lymphomas was frequently reported in the European literature before B. burgdorferi was recognized. These findings suggest a relation between B. burgdorferi infection and cutaneous B cell lymphoma. In geographic regions where infected ticks are present, borreliosis should be considered in patients with cutaneous B cell lymphoma.