OBJECTIVE: To investigate fetal brain development in vivo using early postnatal cranial MRI in term and preterm newborn infants. STUDY DESIGN: 51 infants, 1.5-T whole-body system, extremity coil, spin-echo images obtained in all three planes (T1- and T2-weighted). Independent review by two neuroradiologists (blinded for gestational age and medical history) regarding the development of 12 sulci and 10 gyri using a modified scoring system. Gestational age was obtained either by ultrasound or from the first day of the last menstrual period. Exclusion criteria: head circumference <10th or >90th percentile, cerebral malformation, chromosomal/metabolic disorder. RESULTS AND CONCLUSION: Age at study ranged from 23 5/7-43 0/7 postmenstrual weeks. Brain maturation starts in the central area and proceeds towards the parieto-occipital cortex. The frontal cortex develops last. Transition to degrees 1, degrees 2 and, degrees 3 starts at week 25, 32 and 35 in the central cortex and is completed at week 34, 36 and 39, respectively. Our data compare favourably with the two previously published reports about brain maturation. Early MR imaging seems therefore to be suited to study maturation of the fetal brain. This may be of pathophysiological relevance in neonatal intensive care and neurological follow-up studies.