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The aim of this study was to determine the characteristics of the cognitive function in very-low-birth-weight infants (VLBWI) at 6 years of age and investigate significant factors during neonatal intensive care unit admission that affect cognitive outcomes.One hundred and eighty-nine VLBWI (28.4 weeks, 1024 g), including 93 extremely low-birthweight (EL) infants whose birthweight was <1000 g (26.8 weeks, 759 g) and 96 very low-birthweight (VL) infants whose birthweight was 1000–1499 g (30.0 weeks, 1281 g), were enrolled. The cognitive function was measured using the Wechsler Intelligence Scale for Children version 3, three IQ tests, four factor indices and 13 subtest scores. Regression analyses were performed to analyze the cognitive indices and clinical variables during neonatal intensive care unit admission.The full-scale IQ (FIQ) in the EL infants was 85.3 ± 13.4, which was significantly lower than the 91.8 ± 9.7 observed in the VL infants. The verbal IQ and performance IQ in the EL infants were also lower than those observed in the VL infants. The rate of difference between verbal IQ and performance IQ >14 was 20% in the EL infants and 22% in the VL infants. A multiple linear regression analysis revealed a significant relation between FIQ and HC (P = 0.002) and FIQ and dexamethasone (P = 0.012).In comparison with that observed in the VL infants, the intelligence quotient of the EL infants was inferior and exhibited more inter-individual variation. Intra-individual imbalances of the cognitive function were highly observed irrespective of the EL or VL status. Restriction of intrauterine brain growth and greater doses of dexamethasone may be harmful for subsequent cognitive outcomes.