BACKGROUND Attention-deficit/hyperactivity disorder (ADHD) is a prevalent and disabling lifespan disorder, but little is yet known about risk factors for ADHD persisting beyond adolescence. The present study investigates the association between pregnancy and birth complications and ADHD in adulthood. METHODS We used data from the Medical Birth Registry of Norway to compare pre-and perinatal risk factors among 2323 adults approved for medical treatment for ADHD, with the remaining population born during the same years, 1967-1987, and surviving into adulthood (n = 1,170,073). Relative risks (RR) adjusted for potential confounders were calculated. RESULTS Preterm (< 37 weeks of gestation) and extremely preterm birth (< 28 weeks of gestation) were associated with 1.3- and 5-fold increased risks of ADHD, respectively. Birth weights <2500 g and <1500 g also increased the risk of ADHD (RR: 1.5, 95% confidence interval [CI]: 1.2-1.8, and RR: 2.1, 95% CI: 1.3-3.6, respectively). Five-minute Apgar scores <4 and <7 were associated with 2.8- and 1.5-fold increased risks of persisting ADHD, respectively. Maternal epilepsy (RR: 1.7, 95% CI: 1.1-2.7) and offspring oral cleft (RR: 2.8, 95% CI: 1.6-4.9) occurred more frequently among adult ADHD patients. CONCLUSIONS This is the first population-based study of pre-and perinatal risk factors in adults with ADHD. We show that low birth weight, preterm birth, and low Apgar scores increase the risk of ADHD, persisting up to 40 years after birth. The increased risk of ADHD related to oral cleft and to maternal epilepsy warrants further investigation to explore possible causal mechanisms.