Objective. Transient hypothyroxinemia in very premature infants is associated with developmental problems. A randomized, placebo-controlled trial of thyroxine (T4) supplementation was conducted in a group of 200 infants <30 weeks' gestation. T4 supplementation improved mental outcome at 2 years old in children of 25/26 weeks' gestation only. The effect of T4 supplementation beyond 2 years of age is unknown. We present the effects of neonatal T4 supplementation on outcome at early school age. Methods. Standardized measurements were used to assess cognitive, behavioral, and motor outcome, as well as a qualitative assessment of neurologic functioning. Survivors of the T4trial were assessed at the age of 5.7 years. Results. Ninety-nine percent of the 157 survivors participated. Outcome on all domains was comparable between the T4 group and placebo group. In children <27 weeks' gestation, a 10 IQ point difference was found in favor of the T4 group, whereas in children of 29 weeks' gestation, a difference of 15 IQ points was found in favor of the placebo group. Teachers' reports showed less behavioral problems in the T4-treated children of 25/26 weeks' gestation, but more behavioral problems in the T4-treated children of 27 weeks' gestation. Differences in motor outcome and neurologic outcome were in favor of the T4-treated children <29 weeks' gestation, but not of the T4-treated children of 29 weeks' gestation. Conclusions. We found benefits of T4supplementation for children <29 weeks' gestation, and especially in children of 25/26 weeks' gestation. However, in children of 29 weeks' gestation T4 supplementation is associated with more developmental problems.