OBJECTIVE: Children born at <32 weeks' gestation are at increased risk of intellectual impairment. Few studies have focused on the majority of preterm children born at 32 to 36 weeks' gestation. We aimed to investigate the association between the full range of gestational ages at birth and the risk of not completing basic school. METHODS: This longitudinal, register-based study included all live-born infants in Denmark from 1988 to 1989. Data were obtained from national registers. School achievements were evaluated by using the examination marks. The association between gestational age and not completing basic school was estimated, taking into account the effect of their parents' educational level, being small for gestational age, plurality, and cerebral palsy. RESULTS: The cohort constituted 120 585 infants, of whom 118 281 (98.6%) were alive in 2007. Of these infants, 5.01% (n = 5.928) were born before 37 weeks' gestation. Of the subjects born before 37 weeks' gestation, 11.5% (95% confidence interval: 10.7–12.4) did not complete basic school compared with 7.5% (95% confidence interval: 7.3–7.6) of those born at term. The percentage of subjects who did not complete basic school increased with decreasing gestational age. The increase was steeper at <31 weeks (4.2% per week) than at 31 to 36 weeks' gestation (0.5% per week). CONCLUSIONS: The risk of not completing basic school increased with decreasing gestational age. The risk was moderate at ≥31 weeks' gestation and increased steeply at <31 weeks' gestation. The increase at <31 weeks' gestation was only partly explained by cerebral palsy.