Abstract Background Preterm born very-low-birth-weight (VLBW: birth weight ≤1500 grams) survivors have increased risk of perinatal brain injury that may cause deviant brain development and later neuroimpairments, including reduced cognitive functioning. Aims In this long-term follow up study of three year-cohorts (birth years 1986-88) of VLBW subjects and term born controls with normal birth weight, the aim was to examine differences in brain volumes at age 20 years. In addition, the relationships between brain volumes and cognitive abilities and perinatal variables were explored. Methods Forty-four VLBW subjects and 60 controls were assessed with cognitive testing (Wechsler Adult Intelligence Scale - WAIS-III) and structural MRI at 1.5 Tesla, using the FreeSurfer 5.1 software for volumetric analysis. A subpopulation had MRI performed also at age 15, and for this group changes in brain volumes with age were examined. Results The VLBW subjects had smaller brain volumes, especially of thalamus, globus pallidus and parts of the corpus callosum, and larger lateral ventricles than controls at age 20. However, no significant group differences in longitudinal change from age 15 to 20 were observed. The most immature and smallest VLBW subjects at birth, and those with the highest perinatal morbidity, showed most pronounced volume deviations. Positive associations between several brain volumes and full IQ, as well as three of four IQ indices in the VLBW group, were observed. Conclusion Reduced volumes of grey and white matter and ventricular dilatation in VLBW young adults may indicate permanent effects on brain development from perinatal brain injury with influence on later cognitive function.