The gastrointestinal involvement and immunological findings in 16 patients with Behçet's disease are described. Four of 15 biopsy specimens of jejunal mucosa showed marked lymphangiectasia in the lamina propria similar to the appearance which was thought to be a characteristic finding in protein-losing enteropathy. None of the patients had hypoproteinaemia. Increases in serum immunoglobulin IgA were proved in six of 15 cases; in IgM, five of 15; and in IgG, one of 15. Electron microscopic studies showed that there were thousands of precipitated lymph protein bodies in the extracellular spaces of the lamina propria. Ulcers and healed ulcers of the large intestine were studied by light microscopy.