Introduction Adequate bowel preparation is essential for successful completion of colonoscopy. This study examines whether previous bowel resection affects the quality of bowel preparation. Methods This study prospectively included patients who had gastric or colonic resection (bowel resection group, n = 92) and a control group (n = 92). All patients received 4 L polyethylene glycol (PEG) for bowel preparation. Quality of colonic preparation was assessed using the Aronchick scale (excellent, good, fair, or poor) and was categorized as satisfactory (excellent or good) or unsatisfactory (fair or poor). We analyzed whether previous gastric or colonic resection is associated with unsatisfactory preparation. Results Bowel preparation quality was significantly different between the resection group (0, 39.1, 43.5, and 17.4%, for excellent, good, fair, and poor) and control group (3.3, 53.3, 38.0, and 5.5% for excellent, good, fair, and poor, P = 0.011). Inadequate bowel preparation was significantly higher in the resection group than in the control group (60.9% vs. 43.5%, P = 0.018). Univariate analysis revealed height, weight, body mass index, and bowel resection to be predictors of unsatisfactory preparation. Multivariate analysis revealed bowel resection [odds ratio (OR) 2.12; 95% confidence interval (CI): 1.16-3.86] and obesity (body mass index >= 25 kg/m(2)) (OR 2.16; 95% CI: 1.13-4.12) to be independent predictors of unsatisfactory preparation. The prevalence of unsatisfactory and poor bowel preparation quality was 79.3 and 37.9% in obese patients with previous bowel resection. Conclusions Previous bowel resection was an independent predictor of unsatisfactory PEG bowel preparation. More attention is needed for patients with previous bowel resection, especially for obese patients.