Jejunal mucosal function and structure was examined in 31 patients with ulcerative colitis and 29 patients with Crohn's disease with ileal, ileocolonic or colonic involvement; A significant reduction of the specific activity of disaccharidases (lactase, sucrase and trehalase) in jejunal mucosal homogenate occurred in patients with inflammatory bowel disease. Similarly, alkaline phosphatase was reduced in ulcerative colitis. Several dipeptidases such as glycyl-leucine, leucyl-glycine, glycyl-glycine and valyl-proline hydrolase activities were lower in patients with inflammatory bowel disease than in controls. Histological changes in jejunal mucosal biopsies occurred in 71% of patients with ulcerative colitis and 61% with Crohn's disease. These changes ranged from mild abnormalities of villus architecture to marked reduction of villus height. Most patients with a reduction in mucosal enzymes had concommitant morphological changes in jejunal mucosal biopsy. The results of this study indicate that functional and structural abnormalities of the jejunal mucosa frequently occur in patients with inflammatory bowel disease without radiologic evidence of proximal small bowel involvement.