Peer problems are linked to attention deficit hyperactivity disorder (ADHD) symptoms and the serotonin system is thought to be involved in ADHD‐related behavior. Hence, from a Gene × Environment perspective, the serotonin transporter 5‐HTTLPR may play a moderating role. In two large community samples, the moderating role of 5‐HTTLPR was examined related to more hyperactivity–impulsivity symptoms (HI symptoms) predicted by more peer problems. In Study 1, involving 642 Norwegian children, results indicated that for s‐allele carriers only, caregiver‐reported peer problems at age 4 predicted more parent‐reported HI symptoms at age 6. In Study 2, similar results emerged involving 482 American children. Discussion focuses on differential sensitivity to the adverse effects of poor peer relations.