Differential absorption of D-xylose and 3-0-methyl-D-glucose, and unmediated intestinal permeation (simple diffusion) of lactulose and L-rhamnose, have been investigated in 20 patients with dermatitis herpetiformis. Both iso-osmolar and hyperosmolar test solutions were employed and the results were compared with those obtained from a group of healthy adult volunteers. The findings in each patient have been correlated with small intestinal histology. The majority of patients with villous atrophy had abnormally raised intestinal lactulose permeation and lactulose/rhamnose permeability ratios, whereas patients with normal small intestinal morphological grading did not differ significantly from the healthy control group in this respect. There was a high incidence of delayed plasma D-xylose absorption peaks in dermatitis herpetiformis irrespective of small intestinal histological findings. These results imply that abnormal intestinal permeability in dermatitis herpetiformis is the result of gluten-induced damage to the mucosa rather than an inherent primary defect. It is therefore improbable that the rash in this condition is purely a manifestation of increased intestinal permeation of antigen.