Background: Bariatric surgery may be associated with surgical complications. The aim of the study was to identify significant risk factors for postoperative complications in patients undergoing Roux-en-Y gastric bypass (RYGBP). Methods: The study consisted of 75 consecutive patients undergoing RYGBP. Full medical examination was performed, and the following parameters were assessed in the fasting state: plasma glucose, insulin, leptin, serum lipids, liver function tests, and lipoprotein Lp(a). All subjects had oral 75 g glucose tolerance test before the surgery. All complications occurring within 6 months after the RYGBP were recorded. The patients were divided into Group 1 - patients in whom complications occurred, and Group 2 - patients with no complications in the 6-month period. Results: Postoperative complications occurred in 16 patients (wound infection, hernia, splenic injury, gastro-jejunal obstruction, duodenal ulcer, lower limb deep vein thrombosis). 3 significant risk factors for postoperative complications within 6 months after gastric bypass were found: 1) fasting plasma glucose ≥ 6.0 mmol/l (OR 11.0; 95% confidence interval (CI) 2.1-77.3), 2) age ≥40 years (OR 5.89, 95% CI 1.35-29.4), and 3) BMI ≥45 kg/m² (OR 4.1, 95% CI 1.04-17.2). Conclusion: RYGBP is associated with increased risk of developing early postoperative complications in subjects with even slightly elevated fasting plasma glucose, age ≥40 and BMI ≥45 kg/m².