Context: Hibernoma is a rare benign tumor histologically similar to brown adipose tissue. Some studies reported weight loss in patients with this tumor; however, the mechanisms have never been investigated. Objective: The purpose of this study is to explore the impact of hibernoma resection on the whole-body metabolism. Patient and Methods: A 68-year-old woman was examined after a weight loss of 10 kg in 6 months. Body composition, food intake, physical activity, blood levels of thyroid hormones, and lipid profile were assessed before surgery and during 1 year after surgery. The patient's resting energy expenditure (REE) over time was compared to a control group of 18 matched healthy volunteers. Results: Within 1 year after hibernoma resection, the patient gained 15 kg of body weight. This was associated with fat mass gain (+41%), mainly in the abdominal region (+48%). The patient also developed hepatic nonalcoholic steatosis, mild hypertriglyceridemia, and reduced levels of high-density lipoproteins. REE increased during the dynamic phase of weight gain, compared to the presurgery measurement, and returned to baseline after 1 year. Food intake was increased by 37.5% 6 weeks after resection of the hibernoma and returned to baseline values within 6 months. Conclusions: In our study conditions, hibernoma did not alter REE, but weight gain did. Specific physical activities and dietetic follow-ups are suggested for those patients to prevent excess fat mass gain and metabolic disorders after hibernoma resection. More studies should focus on hibernoma mechanisms inducing weight loss. Affiliations