In two males, 11 and 12 years of age, referred for attention-deficit–hyperactivity disorder (ADHD), 47,XYY syndrome was diagnosed. A team that included a neuropsychologist, a physiotherapist, and a physician examined them. Stature (patients were above 97.5% height for age), muscle consistency, and tremor indicated chromosome analysis. Psychological tests results did not fully fit the ADHD diagnosis. On the basis of our clinical observation we felt that stimulant medication was indicated. Administration of methylphenidate led to improved motor and cognitive functions as well as social adaptation in both patients. We suggest that this treatment might well be considered in clinically similar patients with XYY sex chromosomes; we further suggest that learning problems in such individuals may be related to ADHD.