In patients with signs and symptoms of malabsorption, suggestive of gluten-sensitive enteropathy, small intestinal biopsies sometimes only reveal infiltration of lymphocytes into the mucosal epithelium. This infiltrative lesion (Marsh I) is not a definite proof for gluten-sensitive enteropathy. However, in the present study, we aimed to show that a subgroup of these patients could ultimately be identified as being gluten sensitive.A total of 38 patients with a Marsh I lesion were subjected to a gluten challenge comprising 30 g of gluten added daily to a normal gluten-containing diet for 8 wk. Before and after the challenge, small intestinal biopsies were taken, and symptoms and signs of malabsorption were scored.In 12 patients we demonstrated a significant change in mucosal histopathology, i.e., subtotal villous atrophy (Marsh IIIB, n = 1), partial villous atrophy (Marsh 3A, n = 6) or infiltrative-crypthyperplastic lesions (Marsh II, n = 5). In the other 26 patients, the small intestinal mucosa remained unchanged. After initiation of a gluten-free diet, follow-up small intestinal biopsies in 12 patients who initially had progressive mucosal pathology after gluten challenge showed normalization of mucosal pathology in seven cases, regression to a Marsh I lesion in four, and to a Marsh II lesion in one. Symptom relief was seen in all 12 patients. Ten of 26 patients without histological response to the gluten challenge were motivated to adhere to a gluten-free diet. Follow-up biopsies revealed unchanged Marsh I lesions in eight patients and normalization (Marsh 0) in two patients. Three patients had follow-up biopsies while on a normal diet. All had unchanged Marsh I lesions.In the present study we demonstrated that a gluten challenge might be useful in identifying patients as being sensitive to gluten if initial small intestinal biopsies reveal only minor abnormalities.