The effect of nonsteroidal anti-inflammatory drugs (NSAIDs) on the gastric and duodenal mucosa was studied in 53 patients. On endoscopic biopsies, 87% patients showed gastritis and 90% duodenitis. Chronic superficial gastritis was the most frequent lesion in the stomach (49%); and mild duodenitis in the duodenum (54%). The antrum was the most frequently involved site in the stomach. An interesting finding in cases of duodenitis was the presence of dilated intestinal glands plugged with mucus.