Malignant angioendotheliomatosis is a rare, systemic, usually fatal disease characterized by massive proliferation of large, neoplastic, mononuclear cells within the lumen of small blood vessels. Recent studies suggested that the tumor cells are of lymphoid origin. We studied two cases of malignant angioendotheliomatosis by Southern blot hybridization analysis that showed rearrangements of the immunoglobulin heavy chain and kappa light chain (case 2), indicating the presence of a monoclonal B cell lymphoma. Our results provide further evidence that malignant angioendotheliomatosis is an angiotropic lymphoma.