Objective: To evaluate the acute efficacy of methylphenidate (MPH) in Brazilian male children and adolescents with ADHD. Method: In a 4-day, double-blind, placebo-controlled, randomized, fix dose escalating, parallel-group trial, 36 ADHD children and adolescents were allocated to two groups: MPH (n = 19) and placebo (n = 17). Participants were evaluated pre- and posttreatment using the 10-item Conners Abbreviated Rating Scale (ABRS), the Children's Global Assessment Scale (CGAS), and a simplified version of the Continuous Performance Test (CPT). Results: The MPH group had a significantly greater decrease in ABRS scores and a significantly higher increase in CGAS scores than the placebo group (p less than 0.01). The MPH group showed also a significantly higher proportion of patients with a robust improvement (decrement of at least 50% in the ABRS score after the intervention) than the placebo group ( p less than 0.01). The MPH effect size for the ABRS was 1.05 (95% CI = 0.73-1.37). Conclusion: Our results extend the efficacy of MPH on the ADHD core symptoms extensively demonstrated in clinical trials with samples from some developed countries to a sample from a developing country where a diverse culture may modulate the clinical presentation of the disorder. (Contains 2 tables and 1 figure.)