The clinical and pathological features of 17 "Western" type primary abdominal lymphomas (WTL) are compared with 14 of "Mediterranean" type (MTL). The MTL involved only young adult Mulatto and African patients in whom malabsorption and abdominal pain were the major clinical features. The WTL also predominantly affected Mulatto patients but four cases occurred in Caucasians, and the mean age at presentation was two decades later. An obstructive presentation was the most common; only one patient in this group had evidence of malabsorption. The WTLs were located mainly in the distal small bowel and were nearly all of monomorphic lymphocytic or histiocytic type. The MTLs were sited mainly in the duodenum and jejunum and were of an unusual pleomorphic histologic type. A spectrum of cells from those resembling atypical lymphocytes to large histiocytic types were seen, some of the latter resembling Reed-Sternberg cells. The pattern of mesenteric node infiltration in the MTLs was also unusual in that preservation of the medullary sinuses was a common finding. A notable feature of the MTLs was the presence of a heavy infiltration of mature-looking plasma cells associated with a fairly severe villous atrophy in the lamina propria of the small bowel. In the WTLs the adjacent small bowel did not show this feature. In addition to the above cases three patients with a similar heavy plasma cell infiltrate and villous atrophy but without evidence of a lymphoma are described. These cases may represent examples of MTL in a pre-malignant phase. Also included in this study are three patients with alpha-chain disease (alpha-CD), all with a heavy plasma cell infiltration and villous atrophy of the lamina propria and a pleomorphic type lymphoma involving the mesenteric nodes in all, and the small bowel in two. The lymphomas in alpha-CD have been interpreted as immunoblastic sarcoma by Lukes and Collins. Both genetic and environmental factors may be operative in the MTLs including the cases of alpha-CD.