Objective: Attention-deficit/hyperactivity disorder (ADHD) is a heritable neurodevelopmental disorder characterized by inattention, disorganization, and/or hyperactivity-impulsivity. This study investigated the association between the metabotropic glutamate receptors (GRM) 7 rs3749380 polymorphism genotypes and subjective/objective treatment responses to methylphenidate (MPH) in Korean children with ADHD. Methods: This study enrolled 86 medication-naïve children with ADHD in an open-label 8-week trial of MPH. The subjects were genotyped and then evaluated using the ADHD Rating Scale (ARS), the Continuous Performance Test (CPT) and the Clinical Global Impression Scale (CGI) before and after treatment. Results: After 8-week MPH treatment, children with the GRM7 rs3749380 polymorphism T/T genotype had a different response in terms of visual response times and auditory commission errors on the CPT than C/C or C/T genotype groups. Conclusions: These results suggest that the GRM7 rs3749380 polymorphism is associated with the response of MPH in patients with ADHD. Further studies, including replication of our findings using a control or comparison group and a larger sample, are warranted to evaluate the association between the GRM7 genes and treatment responses to MPH in subjects with ADHD. [Anadolu Psikiyatri Derg 2016; 17(6.000): 442-450]