Growing evidence suggests that telomeres, telomerase, matrix metalloproteinase-9 (MMP-9), andSIRT1(sirtuin1) are involved in the pathophysiology of neuropsychiatric and neurodevelopmental disorders. However, whether these molecules are contributors to attention-deficit/hyperactivity disorder (ADHD) has been little explored and poorly understood. This study aimed to determine the potential role of telomerase, MMP-9, andSIRT1in children with ADHD. The study was performed on 46 children with ADHD aged between 8 and 14 and 43 healthy children matching in age and gender. Children were evaluated by Kiddie-Sads-Present and Lifetime Version, Conners' Parent Rating Scale-Revised Short Form (CPRS-RS) and Stroop test. Serum telomerase, MMP-9, andSIRT1levels were measured by a quantitative sandwich enzyme-linked immunosorbent assay. MMP-9 and telomerase levels were significantly higher andSIRT1levels were significantly lower in patients with ADHD than those of controls. All three molecules were significantly associated with both the severity of ADHD symptoms and cognitive functions. This is the first attempt to indicate that the important role of telomerase, MMP-9, andSIRT1in ADHD, and the association of all these molecules with the severity of ADHD and cognitive functions, but future studies are required to verify these results.