Surgical specimens, postmortem material, or both from 22 patients with the syndrome designated as “Mediterranean abdominal lymphoma with malabsorption” were studied in detail. All patients exhibited a malabsorption syndrome. In most of those carefully followed from the onset of the disease, the syndrome seemed to have preceded the development of palpable abdominal masses. In 20 of the 22 patients, intestinal tissues were available for histologic study. In all specimens, diffuse severe plasma cell infiltrations were evident in the intestinal mucosa and submucosa. Malignant lymphomas in the form of single or multiple circumscribed intestinal tumors occurred in 14 of these 20 patients and malignant lymphomas of the mesenteric lymph nodes in two. In four patients, no malignant lymphomas were evident. These observations suggest that the diffuse plasma cell infiltrations, rather than the malignant lymphomas, were responsible for the malabsorption syndrome. There was no morphological evidence that the malignant lymphomas observed in 75% (16 of 20) of the patients were histogenetically related to the diffuse plasma cell infiltration. The possibility is suggested that this abnormal, though not provably neoplastic, proliferation of plasma cells is a morphological manifestation of an immune deficiency state which predisposes the patients to the development of malignant lymphoreticular neoplasms.