The aim of this study was to compare gastric residual volume (GRV) in patients given a split-dose versus a conventional single-dose of polyethylene glycol (PEG) preparation before undergoing anesthetic colonoscopy. Methods. In a prospective observational study, we assessed GRV in outpatients undergoing same-day anesthetic gastroscopy and colonoscopy between October 8 and December 30 of 2016. Outpatients were assigned to the split-dose (1 L PEG in the prior evening and 1 L PEG 2-4 h before endoscopy) or single-dose (ingestion of 2 L PEG ≥ 6 h before endoscopy) regimen randomly. Bowel cleansing quality was assessed with the Boston Bowel Preparation Scale (BBPS). Results. The median GRV in the split-dose group (17 ml, with a range of 0-50 ml; N = 65) was significantly lower than that in the single-dose group (22 ml, with a range of 0-62 ml; N = 64; p = 0.005), with a better bowel cleansing quality (BBPS score 8.05 ± 0.82 versus 7.64 ± 1.21; p = 0.028). GRV was not associated with diabetes or the use of medications. Conclusions. GRV after a split-dose preparation and fasting for 2-4 hours is not larger than that after a conventional single-dose preparation and fasting for 6-8 hours. The data indicates that the split-dose bowel preparation might not increase the risk of aspiration.