Introduction Bariatric surgery is currently the only method that results in a permanent loss of excess body weight in morbidly obese patients, furthermore it alleviates the course or causes resolution of pre-diabetes and type 2 diabetes mellitus (DM2). Shortly after its implementation, laparoscopic sleeve gastrectomy (LSG) has become the most commonly performed bariatric surgery. It is widely believed that DM2 and the associated chronic hyperglycemia (expressed by the level of glycated hemoglobin - HbAlc) increase the risk of postoperative complications and mortality, what has been confirmed in some branches of surgery. Hence research on its effect on postoperative outcomes after LSG is extremely valuable. There is a lack of studies analyzing the influence of LSG on prediabetes and DM2 at various time intervals after surgery, especially in the perioperative period, what is important from the perspective of patient care. Aim Aims: to evaluate the influence of preoperative glycemic control on postoperative results of LSG; analysis of 24-hours glucose fluctuations over a 10-days perioperative period with the use of continuous glucose monitoring in patients after LSG in comparison with laparoscopic gastric bypass (LRYGB). Material and methods A retrospective, multicenter observational study that enrolled LSG patients to analyze the effect of HbAlc on perioperativ ; e and postoperative morbidity, prolonged length of hospital stay, and hospital re-admissions. A prospective observational study in DM2 patients and control group that were enrolled in either LSG or LRYGB with comparison of the 24-hours glucose fluctuations over a 10-day perioperative period using a continuous glucose monitoring system. Results summary Preoperative HbAlc levels did not significantly increase the chance of developing perioperative and postoperative complications, as well as prolonged hospital stay after LSG. HbAlc >7.3% increased the risk of hospital re-admission. In patients with HbAlc >6.5% who were not treated for DM2, no increased risk of perioperative, postoperative complications, or prolonged hospitalization was observed. Continuous glucose monitoring has been proven useful in bariatric patients in the perioperative period. The period of reactive hyperglycemia was recorded in response to the surgical trauma on the 1st and 2nd postoperative days after LSG and LRYGB. From the 3rd day, significantly lower mean daily glucose levels were observed in patients with DM2 after LRYGB compared to patients after LSG. Regardless of DM2, significantly lower mean daily glucose levels were present in all LRYGB patients at from 7th to 10th day as compared to the LSG.