['The role of the placenta in fetal programming has been recognized as a highly significant, yet often neglected area of study. We investigated placental size in relation to psychopathology, in particular attention deficit hyperactivity disorder (ADHD) symptoms, in children at 8 years of age, and later as adolescents at 16 years.', 'Prospective data were obtained from The Northern Finland Birth Cohort (NFBC) 1986. Placental weight, surface area and birth weight were measured according to standard procedures, within 30 minutes after birth. ADHD symptoms, probable psychiatric disturbance, antisocial disorder and neurotic disorder were assessed at 8 years (n\u200a=\u200a8101), and ADHD symptoms were assessed again at 16 years (n\u200a=\u200a6607), by teachers and parents respectively. We used logistic regression analyses to investigate the association between placental size and mental health outcomes, and controlled for gestational age, birth weight, socio-demographic factors and medical factors, during gestation. There were significant positive associations between placental size (weight, surface area and placental-to-birth-weight ratio) and mental health problems in boys at 8 and 16 years of age. Increased placental weight was linked with overall probable psychiatric disturbance (at 8 y, OR=\u200a1.14 [95% CI=\u200a1.04-1.25]), antisocial behavior (at 8 y, OR\u200a=\u200a1.14 [95% CI=\u200a1.03-1.27]) and ADHD symptoms (inattention-hyperactivity at 16 y, OR=\u200a1.19 [95% CI\u200a=\u200a1.02-1.38]). No significant associations were detected among girls.'] Is placental size associated with mental health in children and adolescents?