Recent guidelines for the diagnosis and treatment of attention deficit hyperactivity disorder (ADHD) have claimed the possible benefits of psychoeducational techniques in the comprehensive management of ADHD. To evaluate the efficacy of a psychoeducation programme for parents of children and adolescents with ADHD in a clinical setting using a blind randomized trial. 81 children/adolescents with ADHD were randomly assigned for their families to receive either a well-structured psychoeducation programme (intervention group, n = 44), or a parent counselling and support intervention (control group, n = 37). Measures of child ADHD symptoms, psychopathology, quality of life and family stress were taken before and after intervention and after a year follow-up. Parents and evaluators were unaware of the condition received. Compared to the support control group, the psychoeducation group showed ADHD Index and cognitive/inattention levels significantly reduced after the intervention ended (Mann-Whitney U = 3.34; p = 0.001; Mann-Whitney U = 3.47; p = 0.001). An improvement in the pro-social domain was also observed after 1 year follow-up (Mann-Whitney U = -2.37; p = 0.018), and clinical global impression found a statistically significant effect for severity over the time. Differences were initially found for the impact of the disorder in the family in different domains, including emotional and social functioning; these differences were no longer significant after alpha correction. No significant differences in quality of life or family stress were found in comparison with the control group. This psychoeducation programme is a valuable treatment for parents/carers of children/adolescents with ADHD, which needs to be considered when evaluating different non-pharmacological treatment options. Psychoeducation and other kind of non-pharmacological approaches need to be regarded not as a substitute, but as a complementary treatment to medications; these approaches might help other very crucial aspects of ADHD including social and familiar outcomes.