It is now well documented that a characteristic mucosal lesion of the proximal small intestine is present in acute nonbacterial gastroenteritis. To determine whether a gastric mucosal lesion also accompanies this illness, stool filtrate containing Norwalk agent was given orally to 15 volunteers after base line biopsies of gastric fundal and/or antral mucosa had been obtained. Gastric fundal and/or antral biopsies were then obtained serially between 24 and 168 hr after administration of the inoculum. Nine volunteers developed symptoms of gastroenteritis. gastric biopsies from those with normal base line fundal and/or antral biopsies remained normal during and after clinical illness. Those volunteers who had mild to moderate gastritis in their base line biopsies showed persistence but no progression of the lesion during illness. In 4 of the volunteers who became ill, intestinal biopsies were available and showed the typical gastroenteritis lesion. These results indicate that acute infectious nonbacterial gastroenteritis induced by Norwalk agent is not associated with histologically detectable gastric mucosal lesion.