It is recognized that the risk of multiple births, preterm birth, and low birth weight is magnified after in vitro fertilization (IVF), but little is known of the long-term outcome for children born after IVF. This retrospective study, using a population-based cohort of 5680 children born after IVF, sought to clarify the risk of severe neurological disorder, mental retardation, and visual defects. The study children, aged 18 months or older at follow-up, were matched for gender, year of birth, and hospital of birth with 11,360 control children. A second set of control subjects, 4120 twins, was chosen for 2060 of the 5680 study children who were twins born after IVF. Children born after IVF were more likely to require rehabilitation for neurological problems than were control subjects; the odds ratio was 1.7. The risk was 1.4 for singleton infants, whereas for post-IVF twins the risk was not greater than in the control twins. The commonest diagnoses were cerebral palsy (most often spastic diplegia or spastic hemiplegia), suspected developmental delay, congenital malformation, mental retardation, chromosomal aberration, and behavioral disorder. The risk of cerebral palsy was increased nearly 4-fold in post-IVF children. The risk remained after adjusting for gestational age and birth weight. Suspected delayed development also was increased 4-fold in the IVF group; singleton infants had about double the risk of control children. Maternal age did not influence the overall risk of neurological sequelae. The eight post-IVF children with severe visual disorders had an approximately 2-fold increase in risk. Although most children born after IVF are perfectly healthy, they nevertheless are at increased risk for neurological disability or handicap, particularly cerebral palsy. A high rate of twin births is part of the explanation, as are low birth weight and low gestational age. It is estimated that having a single embryo would lower the prematurity rate by approximately 60%. The authors believe that, as a practical clinical matter, only one IVF embryo should be transferred in most cases.