Common variable immunode®ciency (CVID) is a primary immunode®ciency disease characterized by low serum immunoglobulins IgG, IgA, and usually IgM. The central immune de®ciency is impaired secretion of immunoglobulins and lack of antibody production; however, T cell dysfunction and a variety of in¯ammatory complications suggest global immune dysregulation. A number of reports have documented the association of primary immunode®ciency diseases with the development of non-Hodgkin's lymphoma (NHL). In CVID, the risk has been estimated to lie between 1.4% and 7%. As for NHL arising in other immunode®ciency states, the lymphomas in CVID are extranodal and are usually B cell in type. Of 22 B cell lymphomas that have appeared over a period of 25 years in a cohort of subjects with CVID, ®ve lymphomas, appearing in more recently studied subjects, that arose in mucosal sites would be classi®ed as mucosaassociated lymphoid tissue (MALT) lymphomas. MALT lymphomas are low-grade B cell lymphomas that result from a proliferation of neoplastic marginal-zone related cells of lymphoid tissue and tend to occur in organs that have acquired lymphoid tissue due to long-term infectious or autoimmune stimulation. Lymphomas of this kind have not been described in patients with congenital immunode®ciency, although chronic mucosal antigen stimulation is an integral part of these immune de®ciency states. Am.