Mannitol (molecular weight 182), lactulose (342), and polyethylene glycol 400 (range 242-550) absorption was studied in 25 controls, 22 untreated celiacs, and 13 treated celiacs. Untreated celiacs absorbed less mannitol and more lactulose than controls. Absorption of higher as well as lower molecular-weight polyethylene glycols was reduced in untreated celiac disease. Absorption returned towards normal on treatment. Polyethylene glycol and lactulose absorption was enhanced by administering them in a hypertonic solution. Polyethylene glycol 400 but not lactulose or mannitol was lipid soluble in vitro. It was concluded that the mucosa in untreated celiac disease was more "leaky" than normal. Polyethylene glycol 400 absorption data suggested that its absorption may largely be determined by its lipid solubility and was decreased in celiac disease because of the reduced surface area of the small intestine. Polyethylene glycol 400 cannot be recommended as a suitable marker for permeability studies of the small intestine.