Gastrointestinal absorption of orally administered polyethyleneglycols in the 634-1294 dalton range (PEG 1000) was investigated in 44 patients with ileocaecal Crohn's disease and in 24 healthy controls. Absorption of PEG was inversely proportional to molecular weight in both groups. In the Crohn patients the absorption of all tested molecular weights of PEG was significantly less than in the controls. In Crohn's disease there was also less of selectivity of the mucosal barrier against the larger molecules. The state of the distal ileum--acutely inflamed, in remission or post-resection did not influence absorption. Decreased PEG absorption in ileocaecal Crohn's disease probably reflects an abnormality of jejunal mucosal permeability. The observations support the concept that Crohn's disease is a diffuse lesion, affecting the whole gastrointestinal tract. The nondependence of the abnormality on the severity of distal inflammatory changes or on previous resection suggests that abnormal jejunal permeability may be a primary expression of Crohn's disease.