Clinical and genetic studies were analyzed in 47 patients with rheumatoid arthritis (RA) who had upper gastrointestinal (GI) endoscopies. Fifty-three percent of patients with RA had peptic ulcers and/or erosions. Sixty percent of patients with ulcers and/or erosions had a history of peptic ulcer disease. Although a greater number of patients with ulcers and/or erosions was taking regular aspirin or indomethacin, comparable numbers of patients with abnormal and normal endoscopies were using nonsteroidal antiinflammatory drugs. Nineteen of the 25 patients (76%) with ulcers and/or erosions had type O blood. Patients with abnormal and normal endoscopies had similar frequencies of GI complaints and fecal blood loss. GI symptoms and occult fecal blood loss, therefore, are not prominent features of upper GI disease in RA. ABO screening may be helpful in determining which patients with RA are at risk for developing peptic ulcers and/or erosions.