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Aim To investigate the ability of the Bayley Scales of Infant and Toddler Development-Third Edition (Bayley-III), scores to predict later Wechsler Intelligence Scale for Children-Fourth Edition (WISC-IV), performances in a cohort of children born extremely preterm. Methods 323 children, born <27 gestational weeks, were tested with the Bayley-III at corrected age 2.5 years and with the WISC-IV at 6.5 years. Regression analyses investigated the association between Bayley-III scores and WISC-IV full-scale intelligence quotient (IQ). The ability of Bayley-III Cognitive Index scores to predict low IQ was evaluated using receiver operating characteristic curves. Results Bayley-III Cognitive Index scores and IQ had a moderately positive correlation and accounted for 38% of the IQ variance. Using a Bayley-III cut-off score of 70, the sensitivity to detect children with IQ<70 was 18%, and false positive rate was 7%. A Bayley-III cut-off score of 85 corresponded to sensitivity and false positive rates of 44% and 7%, respectively. Conclusions Results emphasise the relative importance of Bayley-III Cognitive Index scores as predictors of IQ. An 85 score cut-off for suspecting subnormal IQ is supported. A less conservative threshold would increase identification of true cases yet increase the risk of wrongly diagnosing children.