Background: Sterile water has been recommended in the water bottle for endoscopic equipment given the concern for infection risk, although studies evaluating prevalence of contamination of the water bottle with clinical outcomes have not been performed. Methods : Over a 13 week period in three endoscopy rooms at a university teaching hospital, the water bottles were filled on a weekly schedule with either sterile (one room) or tap water. The water bottles were sterilized weekly using a Steris®. At the end of each week, an aliquot of the remaining water was placed in a sterile container and quantitative cultures for aerobic and facultative anaerobic bacteria were performed using a 0.01ml calibrated loop according to standard protocol.Cultures were performed in a blinded fashion without knowledge of the water source. Follow-up was performed on all patients within two weeks of the procedure to determine any potential infectious complications. Results: During the study period, 437 procedures were performed (203 endoscopy, 68 colonoscopy, 38 sigmoidoscopy, 128 ERCP). Of a total of 33 cultures, 9 (27%) were positive including 3 bottles where sterile water was used. Bacterial isolates included 5 Flavobacterium sp., 4 Acinetobacter sp., 2 Pseudomonas sp., and 1 Stenotrophomonas maltophilia. Colony counts ranged from 900 to 10,000 per ml. On followup, no patient developed an infection from one of these pathogens. Conclusions : 1. Bacterial growth in the water bottle occurs infrequently, consists predominantly of non-pathogenic organisms,and was not associated with clinical complications. 2. Use of tap water may decrease endoscopy unit costs.