One hundred and seven consecutive patients with hematemesis and/or melena and a diagnosis of duodenal, gastric, or esophageal ulcers were interviewed immediately before or after endoscopy about the use of non-steroid anti-inflammatory drugs (NSAIDs) and symptoms before the hemorrhage. If the patients admitted no symptoms of abdominal pain or discomfort, nausea, vomiting, or heartburn, they were classified as having no ulcer symptoms before the hemorrhage. Patients who had not taken NSAIDs during the last 48 h before the hemorrhage were classified as not having taken NSAIDs. Significantly fewer patients had ulcer symptoms in the group that had used NSAIDs than in the other group (p less than 0.01). This may be interpreted as a possible masking effect by NSAIDs on ulcer symptoms. Physicians and patients should be aware of this possible effect of NSAIDs.