Abstract Disclosure: C. Elatrash: None. S.R. Sisley: None. Background: Hyperphagia, characterized by excessive hunger, can significantly contribute to weight gain in early-onset childhood obesity and lacks an FDA-approved therapy. Stimulants commonly used in the treatment of attention deficit and hyperactivity disorder (ADHD) have a significant side effect of suppressing appetite. However, there is no data on the effectiveness of stimulants to reduce measures of obesity in children with severe obesity and hyperphagia. Methods: We retrospectively analyzed electronic medical records of children < 17 years old with severe, early-onset obesity (defined as a Body Mass Index (BMI) ≥ 120% of the 95th percentile (BMI%95th) prior to age 5 years) who were seen in the Genetic Disorders of Obesity Program (GDOP) at Texas Children’s Hospital (TCH). We included children who had a history of abnormal food behavior consistent with a clinical diagnosis of hyperphagia by having 2 of 9 symptoms of hyperphagia (consuming large portions, consistently feeling hungry, not feeling full with one serving, temper tantrums over food, asking for food frequently, sneaking food, getting up at night to eat, hiding food, constant asking for snacks) and were prescribed lisdexamfetamine treatment for hyperphagia for 6 months. We extracted BMI %95th for up to 12 months prior to treatment and 6 months after treatment. Paired t-tests were utilized to compare data from pre- vs. post-treatment. Results: Our cohort included 22 patients and was 68.2% female, 50% non-Hispanic White, 36.4% Hispanic white, and 13.6% black, with a mean age of 7.73 years±3.82 (range 3-17 years). There was a significant decrease in BMI%95th after treatment with lisdexamfetamine (167.3±38.8 vs. 158.0±40.2; p<0.001). We also observed a significant change in the rate of weight gain after lisdexamfetamine treatment; prior to treatment, patients gained 0.6 BMI%95th points/month compared to a loss of 1.9 points/month after treatment (p=0.002). 16 (72.72%) patients reported tolerable side effects, the rate of side effects was (33)33.33%. 19(57.57%) lasted from 1 week to 1 month and 6(18.18%) lasted more than 1 month. Side effects included adverse behavioral (9)27.27%, sleep disturbance (5)15.15%, and headache (4)12.12%. Conclusion: Children with severe obesity and clinical hyperphagia showed reductions in BMI%95th after six months of lisdexamfetamine treatment. Future research is needed to confirm the clinical efficacy of lisdexamfetamine in this high-risk population. Keywords: Childhood obesity, Stimulants, Hyperphagia, Pharmacologic Treatment Presentation: 6/3/2024