OBJECTIVE: Hypothalamic obesity (HO) is a severe condition following childhood craniopharyngioma (cCP) treatment. Despite multidisciplinary and centralized care, severe Body Mass Index (BMI) changes are still often encountered after cCP surgery. We aimed to perform an in-depth analysis of BMI trajectories in our cCP cohort in the first year post-surgery and to identify characteristics associated with BMI change. METHODS: Data was collected of cCP patients diagnosed since January 2018. Change (∆) in BMI standard deviation scores (SDS) from treatment to 3, 6, 9, and 12 months after and latest follow-up was assessed. Secondary outcomes were patient, tumor, and treatment characteristics associated with ∆BMI-SDS, and interventions for post-operative weight gain. RESULTS: Of the 35 cCP patients, the most significant BMI SDS change was observed within the first three months following cCP resection, with almost half of patients developing an increase of ≥1.0 BMI SDS. Of patients with the most severe BMI change within the 1st three months, 87% were overweight or obese at 12 months after cCP resection. Pharmacological intervention for obesity started on average 10.8 months post-resection. Development of arginine vasopressine deficiency was positively correlated with ∆BMI SDS in the first three months (p=.017). Barriers for obesity intervention in the first months following treatment are discussed. CONCLUSIONS: Following cCP resection, BMI increase due to hypothalamic damage is most severe already in the first three months post-operatively. Post-operative rapid weight gain should be acknowledged as a consequence of hypothalamic damage and requires more early intervention to prevent the development of HO.