IMPORTANCE: Antenatal magnesium sulfate given to pregnant women at imminent risk of very preterm delivery reduces the risk of cerebral palsy in early childhood, although its effects into school age have not been reported from randomized trials. OBJECTIVE: To determine the association between exposure to antenatal magnesium sulfate and neurological, cognitive, academic, and behavioral outcomes at school age. DESIGN, SETTING, AND PARTICIPANTS: The ACTOMgSO4 was a randomized clinical trial conducted in 16 centers in Australia and New Zealand, comparing magnesium sulfate with placebo given to pregnant women (n = 535 magnesium; n = 527 placebo) for whom imminent birth was planned or expected before 30 weeks' gestation. Children who survived from the 14 centers who participated in the school-age follow-up (n = 443 magnesium; n = 424 placebo) were invited for an assessment at 6 to 11 years of age between 2005 and 2011. MAIN OUTCOMES AND MEASURES: Mortality, cerebral palsy, motor function, IQ, basic academic skills, attention and executive function, behavior, growth, and functional outcomes. Main analyses were imputed for missing data. RESULTS: There were 1255 fetuses known to be alive at randomization. Of 867 survivors available for follow-up, outcomes at school age (corrected age 6-11 years) were determined for 669 (77%). There was little difference between groups on any of the cognitive, behavioral, growth, or functional outcomes. (Table Presented) CONCLUSIONS AND RELEVANCE: Magnesium sulfate given to pregnant women at imminent risk of birth before 30 weeks' gestation was not associated with neurological, cognitive, behavioral, growth, or functional outcomes in their children at school age, although a mortality advantage cannot be excluded. The lack of long-term benefit requires confirmation in additional studies. TRIAL REGISTRATION: anzctr.org.au Identifier: ACTRN12606000252516.