Histological observation of the gastroduodenal mucosa in 54 patients with Crohn's disease is reported. The medical or surgical history was available in all patients. Biopsy specimens collected during gastroduodenal endoscopy were fixed in 10% buffered formalin, processed routinely and stained with HE, paS/AB, and modified Giemsa stain and assessed microscopically. Patchy/focal active gastritis or duodenitis was the most consistent microscopic finding. About 1/4 of patients with gastric biopsies presented crypt atrophy, lymphoid tissue proliferation, fibrosis of the lamina propria, foveolar hyperplasia, intestinal metaplasia, erosions or ulcerations. Erosions/ulcers, villous atrophy, fibrosis, lymphoid tissue proliferation and gastric metaplasia were seen in further 34% of patients with duodenal biopsies. Granulomas were detected in 14% of gastric and in 12% of duodenal mucosal samples. Helicobacter pylori infection affected 35% of patients with gastric biopsies, 24% of those with active focal gastritis, and was found in 7/10 foci of gastric metaplasia in the duodenum. Normal gastric mucosa was present in 11% and normal duodenal mucosa in 12% of patients. In our opinion irregularly distributed foci of inflammatory lesions with crypt abscesses, focal fibrosis and/or lymphoid tissue proliferation in the lamina propria of gastroduodenal biopsies, particularly in the absence of H. pylori infection are most important lesions in the diagnosis of Crohn's disease. Epithelioid granulomas are less common and other aetiologies should be excluded.