To assess the risk of infective endocarditis for susceptible patients having upper gastrointestinal (GI) endoscopy, we have prospectively studied the incidence, level, duration and source of endoscopy-related bacteraemia and the microorganisms involved. Blood was drawn for aerobic and anaerobic culture and pour plate estimation from 50 patients undergoing upper GI endoscopy. Blood cultures were positive in two patients with the isolation of Streptococci and other oropharyngeal organisms during and 5 min after endoscopy. Pre-endoscopy salivary cultures in both patients grew Streptococci identical to those isolated from the blood; pour plates were negative. We conclude that though the risk of infective endocarditis after upper GI endoscopy is probably very low, the finding of a 4% incidence of transient low-level bacteraemia of oropharyngeal origin argues for antibiotic prophylaxis for susceptible patients.