The incidence of bacteremia associated with endoscopy varies widely for different endoscopic procedures. This study prospectively evaluates the incidence of bacteremia in patients undergoing elective endoscopic laser therapy for two diseases of the upper gastrointestinal tract. In five patients (eight treatment sessions) with arteriovenous malformations, no bacteremia developed. In 15 patients (26 treatment sessions) with esophagogastric carcinoma, bacteremia developed at some time during the procedure in six (40%). However, bacteremia appeared to be associated with the endoscopic insertion through the tumor rather than the laser treatment per se. These findings lead us to conclude that for patients undergoing endoscopy and laser therapy for esophagogastric malignancy, but not for those being treated for arteriovenous malformations, antibiotic prophylaxis to prevent endocarditis may be indicated.