Introduction: Roux-en-Y gastric bypass (RYGB) is considered the gold standard bariatric procedure with documented safety and effectiveness. Laparoscopic sleeve gastrectomy (LSG) is a newer procedure being done with increasing fre - quency. Randomized comparisons of LSG and other bariatric procedures are limited. We present the results of the first prospective randomized trial comparing LSG and RYGB in the Polish population. Aim: To assess the efficacy and safety of LSG versus RYGB in the treatment of morbid obesity and obesity-related comorbidities. Material and methods: Seventy-two morbidly obese patients were randomized to RYGB (36 patients) or LSG (36 patients). Both groups were comparable regarding age, gender, body mass index (BMI) and comorbidities. The fol - low-up period was at least 12 months. Baseline and 6 and 12 month outcomes were analyzed including assessment of percent excess weight lost (%EWL), reduction in BMI, morbidity (minor, major, early and late complications), mor - tality, reoperations, comorbidities and nutritional deficiencies. Results: There was no 30-day mortality and no significant difference in major complication rate (0% after RYGB and 8.3% after LSG, p > 0.05) or minor complication rate (16.6% after RYGB and 10.1% after LSG, p > 0.05). There were no early reoperations after RYGB and 2 after LSG (5.5%) (p > 0.05). Weight loss was significant after RYGB and LSG but there was no difference between both groups at 6 and 12 months of follow-up. At 12 months %EWL in RYGB and LSG groups reached 64.2% and 67.6% respectively (p > 0.05). There was no significant difference in the overall prevalence of comorbidities and nutritional deficiencies. Conclusions: Both LSG and RYGB produce significant weight loss at 6 and 12 months after surgery. The procedures are equally effective with regard to %EWL, reduction in BMI and amelioration of comorbidities at 6 and 12 months of fol - low-up. Laparoscopic sleeve gastrectomy and RYGB are comparably safe techniques with no significant differences in minor and major complication rates at 6 and 12 months.