We determined in a prospective observational study of 300 consecutive, mechanically ventilated patients the activity of H. pylori infection and the incidence of stress ulcer-related upper gastrointestinal bleeding over time. H. pylori infection was detected by Laser-Assisted Ratio Analyzer (LARA)- (13)C-urea breath test (Alimenterics, Inc., NJ) and serology. Stress ulcer prophylaxis was not prescribed. Endoscopy was performed in cases of upper gastrointestinal bleeding.