Polyethylene glycol (PEG) was used as a probe molecule to investigate intestinal absorption in eight patients with eczema and evidence of food allergy and ten with eczema alone. In both groups absorption of molecules of larger molecular weight was greater than in normal subjects but absorption of molecules of low molecular weight was normal. There was no difference in absorption between eczema patients with or without food allergy. These results suggest that there is an intestinal mucosal defect in eczema which exists whether or not there is coexistent food allergy. Half the patients with eczema alone and two of the eight with food allergy had more of the large molecular weight PEG recovered in their urine in the second 12 h after ingestion than in the first 12 h. This could be the result of abnormal permeability in the more distal small bowel or even in the colon.