Tehran University of Medical Sciences Iranian Journal of Psychiatry 1735-4587 19 1 2023 12 16 99 106 EN Abbas Seymari Department of Psychology, Faculty of Humanities, University of Guilan, Rasht, Iran Ashkan Naseh Department of Psychology, Faculty of Humanities, University of Guilan, Rasht, Iran Sajjad Rezaei Department of Psychology, Faculty of Humanities, University of Guilan, Rasht, Iran Zivar Salehi Department of Biology, Faculty of Science, University of Guilan, Rasht, Iran Maryam Kousha Kavosh Cognitive Behavior Sciences and Addiction Research Center, Department of Psychiatry, School of Medicine, Guilan University of Medical Sciences, Rasht, Iran 2023 01 09 2023 06 21 Objective: This research investigates the alleles of Variable Number of Tandem Repeats (VNTR) intron 8 of the gene SLC6A3 with attention-deficit / hyperactivity disorder (ADHD) in children and adolescents. Method: The study’s target population consisted of children and adolescents referred to the specialized clinic, as well as students attending school in Rasht city during 2021-2022. A sample of 95 children between the ages of 6 and 10 with ADHD was selected as the ADHD group, and 95 healthy children were selected as the control group using purposive sampling. The subjects completed the Child Symptom Inventory-4 (CSI-4) checklist after a clinical interview, and demographic information was collected. Genetic sampling was carried out through hair follicles. The sequence of interest was proliferated using the Polymerase Chain Reaction technique )PCR(; afterward, the samples were used for genotype identification on polyacrylamide gel electrophoresis. Results: The chi-square test results indicated that the 5R / 5R genotype (P = 0.026, χ2 = 7.26) and the 5R allele (P = 0.002, χ2 = 9.35) had a higher frequency compared to the control group. Additionally, the odds ratio test indicated that, compared to other genotypes and alleles, the 5R / 5R genotype (OR = 2.75, 95% CI = 1.29-5.82, P = 0.01) and the 5R allele (OR = 2.02, 95% CI = 1.28-3.19, P = 0.002) increase the odds of developing ADHD by 2.7 and 2 times higher, respectively. Conclusion: The present study successfully showed the association between intron 8 gene polymorphism, which is responsible for encoding the dopamine transporter as well as ADHD in children and adolescents in Iran. https://ijps.tums.ac.ir/index.php/ijps/article/view/3489