Sixty-eight patients with dermatitis herpetiformis underwent jejunal suction biopsies and/or multiple endoscopic duodenal biopsies to evaluate the incidence of small bowel mucosal atrophy and to compare the diagnostic yield of the two methods. Small bowel function tests were also performed to evaluate the extent of functional impairment. Small bowel lesions were observed in 89.4% of jejunal suction biopsies and in 100% of endoscopic duodenal biopsies. Of the 10 patients who underwent both procedures, one had lesions only in the duodenum, one had more severe lesions in the duodenum than in the jejunum, while the remaining 8 patients showed identical lesions at both sites. The 1-h blood d-xylose test after a dose of 5 g proved more sensitive than xylosuria or serum folic acid assay in detecting subclinical malabsorption. Finally, histological features of gluten-sensitive enteropathy can be found in nearly 100% of patients with dermatitis herpetiformis. Upper gastrointestinal endoscopy with duodenal biopsies is at least as sensitive as jejunal suction biopsy in assessing small bowel involvement in dermatitis herpetiformis.