We studied the local humoral immunity of histologically non-involved jejunum in patients with Crohn's disease. Normal subjects and patients with ulcerative colitis served as controls. Jejunal fluid samples were collected during segmental jejunal perfusion, under an occluding balloon and the in vivo jejunal secretion of the following proteins was determined: albumin, orosomucoid, transferrin, alpha 2-macroglobulin, secretory component, monomeric and polymeric IgA, IgG, and IgM. The densities and number of IgA-, IgG-, and IgM-containing cells in the lamina propria of the jejunum were measured on perendoscopic biopsies. Jejunal secretion of polymeric IgA and the density of IgA-containing plasma cells in the lamina propria were significantly lower in patients with Crohn's disease than in both control groups. This abnormal intestinal immune response, which was not correlated to the activity of the disease, might be involved in its pathogenesis.