Obesity is an epidemic disease that is believed to link to other pathologies as well as life-threatening cardiovascular pathology. Here we report a case of monozygotic twins who successfully lost weight following laparoscopic sleeve gastrectomy at the end of the 18-month follow-up period. We aimed to determine the factors affecting the weight loss outcome after sleeve gastrectomy in monozygotic twins. The twins' initial BMIs were 37.1 kg/m2 and 40.2 kg/m2, respectively. Twin A's excess weight loss was 48.4%, 61.3%, 80.6%, 96.8%, and 112.9% at three, six, nine, 12 and 18 months, while Twin B's in the third, sixth, ninth, 12th and 18th months was 23.1%, 41%, 51.3%, 61.5% and 71.8%. On the third, sixth, ninth, 12th, and 18th months of Twin A, the total weight loss was 15.8%, 20%, 26.3%, 31.6%, and 36.8%. In Twin B in the third, sixth, ninth, 12th, and 18th months, it was 8.7%, 15.5%, 19.4%, 23.3%, and 27.2%. When the twins were compared in terms of excess weight loss and total weight loss at 18 months, Twin A was more successful than Twin B. Especially at this point, Twin B's having a child (three years old) at a young age, her low compliance with the recommendations in the post-operative period and her difficulty in changing her lifestyle, environmental factors are as important as hereditary factors in achieving weight loss and a healthy body mass index (BMI) range., Competing Interests: The authors have declared that no competing interests exist., (Copyright © 2023, Huseynov et al.)