The pathology of 25 cases of Mediterranean abdominal lymphoma, better designated as immunoproliferative small intestinal disease (IPSID), are reported from the American University of Beirut Hospital. The series includes nine cases with documented alpha heavy chain disease (alpha-HCD). The disease is characterized by the presence of a diffuse and compact bandlike lymphoplasmacytic infiltration of the proximal small intestinal mucosa. The presence of a concomitant malignant lymphoma in the intestine and/or mesenteric lymph nodes, and of alpha-heavy protein in the serum is commonly encountered. Two histopathologic variants of IPSID are present. The first is characterized by the diffuse infiltration of the mucosa, at sites away from tumoral masses, by either pure plasmacytic infiltration, or mixed lymphoplasmacytic infiltration. This variety is associated with the immunoblastic sarcoma type of malignant lymphoma, and with alpha chain disease (alpha-HCD). The second variant is characterized by a diffuse follicular lymphoid hyperplasia pattern in the small intestinal mucosa. The associated malignant lymphoma is diffuse and undifferentiated often having a starry-sky pattern. This variety is not associated with alpha-HCD. Both histologic variants share the same clinical antecedents. In five patients, mesenteric lymph nodes harbored immunoblastic sarcoma while the intestinal mucosae of the same patients were involved with a benign appearing lymphoplasmacytic infiltration. This finding stresses the need for staging laparatomy. Three patients, with alpha-HCD, had peripheral lymph node involvement with immunoblastic sarcoma. The disease apparently evolves in two stages: an immunoproliferative phase, probably reversible, and a later development of malignant lymphoma. The term immunoproliferative small intestinal disease accurately describes the nature of the entity.