Objective: To estimate the direct costs of treating excess body weight in children and adolescents attending a public children's hospital. Methods: This study analyzed the costs of the disease within the Brazilian Unified Health System (SUS) for 2,221 patients with excess body weight using a microcosting approach. The costs included operational expenses, consultations, and laboratory and imaging tests obtained from medical records for the period from 2009 to 2019. Healthcare expenses were obtained from the Table of Procedures, Medications, Orthoses/Prostheses, and Special Materials of SUS and from the hospital's finance department. Results: Medical consultations accounted for 50.6% (R$703,503.00) of the total cost (R $1,388,449.40) of treatment over the period investigated. The cost of treating excess body weight was 11.8 times higher for children aged 5-18 years compared to children aged 2-5 years over the same period. Additionally, the cost of treating obesity was approximately 4.0 and 6.3 times higher than the cost of treating overweight children aged 2-5 and 5-18 years, respectively. Conclusion: The average annual cost of treating excess body weight was R$138,845.00. Weight status and age influenced the cost of treating this disease, with higher costs being observed for individuals with obesity and children over 5 years of age. Additionally, the important deficit in reimbursement by SUS and the small number of other health professionals highlight the need for restructuring this treatment model to ensure its effectiveness, including a substantial increase in government investment. [ABSTRACT FROM AUTHOR]