Objective: To perform a cost-utility analysis on the treatment of attention deficit hyperactivity disorder (ADHD) with methylphenidate immediate-release (MPH-IR) in children and adolescents from Brazil. Method: A Markov model was constructed to compare MPH-IR vs. no treatment. A 24-week naturalistic study was conducted to collect transition probabilities and utility data. Effectiveness was expressed as quality-adjusted life-years (QALY), and costs reported in 2014 international dollars (I$). The perspective was the Brazilian Unified Health System as payer, and the time horizon was 6 years. Results: Of 171 patients, 73 provided information at baseline, and 56 at week 24. Considering the MPH-IR monthly cost of I$ 38, the incremental cost-effectiveness ratio (ICER) of treatment was I$ 9,103/QALY for children and I$ 11,883/QALY for adolescents. In two-way sensitivity analysis, considering one Gross National Product per capita (I$ 11,530) as willingness-to-pay, a cost of no-treatment lower than I$ 45/month would render MPH-IR a cost-saving strategy. Discussion: MPH-IR treatment of children and adolescents is cost-effective for ADHD patients from the Brazilian public health system perspective. Both patients and the healthcare system might benefit from such a strategy. Trial registration number: NCT01705613.