We have investigated gastrointestinal permeability in children with atopic eczema by measuring the relative urinary excretion rates of the inert di- and monosaccharides lactulose and rhamnose following their oral administration. The median lactulose/rhamnose ratio was greater in 26 children with atopic eczema than in a control group of 29 children which included both healthy individuals and others with various noneczematous dermatoses. This increased permeability may be a primary abnormality of the gut or may reflect intestinal mucosal damage caused by local hypersensitivity reactions to food antigens.