Attention deficit and hyperactivity disorder (ADHD) is a neurobiological disorder frequent in childhood. The main symptoms are attention disorder and/or impulsivity and/or hyperactivity. There are different subtypes of ADHD according to the degree of presence of these three symptoms. There are different therapeutic approaches with high proved effectiveness. Methylphenidate, a stimulant that acts through the dopaminergic and adrenergic pathways, is commonly used for the treatment of ADHD. The Strengths and Difficulties Questionnaire (SDQ) is a brief behavioural screening instrument internationally used for the screening of mental health problems in children and adolescents. It consists in a 25 items questionnaire with 5 different scales: emotional symptoms, conduct problems, hyperactivity / inattention, peer relationship problems and prosocial behaviours. The SDQ score was collected in a sample of ADHD patients with an age between 7 and 12 years. The score obtained before starting treatment with methylphenidate was compared before and after starting treatment, every 3-6 months and up to a period of 2 years. Statistical processing was performed using R, which is a free program for statistical and graphical analysis, that allows temporary analysis. The results indicate that hyperactivity improves throughout the first year of treatment, emotional symptoms and behavioral problems improve during the first 6 months of treatment, pro-social symptoms slowly improve over 2 years. Problems with partners do not improve in the analyzed time.