Intestinal permeability to different-sized polyethylene glycols in Crohn's disease of the colon was compared with that in ileal Crohn's disease and in controls without inflammatory bowel affection. The permeability was assessed both after ingestion of the marker (oral load) and after deposition in the colon during colonoscopy (colonic load). After oral load the absorption was least in the patients with colonic Crohn's disease, intermediate in ileal disease, and greatest in the controls. After colonic load, however, the values were highest in colonic Crohn's disease. The study indicated that in Crohn's disease of the colon there is abnormal permeability in apparently uninvolved proximal small intestine as well as in the colon. Since oral load tests preferentially reflect the absorptive properties of the proximal small bowel, regional tests of absorption are important when the aim is to assess the permeability of the distal small intestine or the colon.