Four young women presented with diarrhea, malabsorption, and intestinal pseudoobstruction. Intestinal biopsy specimens (both peroral and full-thickness) showed flat small intestinal mucosa, sparsity of crypts, and a widespread lymphoid infiltrate in the lamina propria, muscularis propria, and myenteric plexus. There was no neuron or nerve fiber loss or damage in the plexus; muscle cell absence in the vicinity of lymphoid cell infiltration in the muscularis propria probably accounted for the pathogenesis of pseudoobstruction. Immunochemical stains showed that the infiltrate was polyclonal, and none of the patients has developed lymphoma on clinical follow-up of 4-16 yr. Transient improvement in symptoms occurred after antibiotic therapy in 3 patients, and 1 patient had improvement after treatment with cyclophosphamide and prednisone; however, symptoms of pseudoobstruction persist in all. These cases illustrate yet another cause of intestinal pseudoobstruction which is histologically distinct from visceral myopathies and neuropathies. The pathogenesis of this illness may be related to that of diffuse immunoproliferative diseases seen in Third World countries.