Objective: To investigate the interaction of treatment-related hemodynamic changes with genotype status for Synaptosomal associated protein 25 (SNAP-25) gene in participants with attention deficit hyperactivity disorder (ADHD) on and off  single dose short-acting methylphenidate treatment with functional near-infrared spectroscopy (fNIRS). Method: A total of 15 right-handed adults and 16 right-handed children with DSM-IV diagnosis of ADHD were evaluated. Ten milligrams of short-acting methylphenidate was administered in a crossover design. Results: Participants with SNAP-25 DdeI T/T genotype had decreased right deoxyhemoglobin () with treatment. SNAP-25 MnlI genotype was also associated with right deoxyhemoglobin () and changes as well as left change. When the combinations of these genotypes were taken into account, the participants with genotype had increased right with MPH use whereas the participants with or genotypes had decreased right prefrontal . Conclusions: These results suggested that SNAP-25 polymorphism might be associated with methylphenidate induced brain hemodynamic changes in ADHD participants. Keywords: functional near-infrared spectroscopy, ADHD, methylphenidate, SNAP-25