The differentiation of the telencephalon (TC) gyri and the myelination of TC and diencephalon were analyzed quantitatively in 50 children, 32-240 weeks' postconceptional age (PCA). For this purpose, a four-stage grading system was developed by correlating magnetic resonance images from age-matched postmortem brains with subsequently prepared myelin-stained macroslices. Gyration and myelination progressed in a occipito-rostral direction. Stages 3 and 4 myelination were recognized earlier on T1-weighted images [spin echo (SE) 500/30] than on T2-weighted images (SE 3,000/120). However, long repetition time/echo time sequences yielded better contrast and allowed finer differentiation between myelinated and nonmyelinated structures. At term, the posterior limb of the internal capsule was generally myelinated (stage 2), whereas in 50% of the children, myelin appeared in the TC between 55 and 65 weeks' PCA, e.g., between 3 and 6 months postnatally. The morphological differentiation of the brain surface preceded the biochemical maturation (myelination) by an average of 12 months. Applying such a grading system to a larger population will help establish normal standards for brain maturation and a "brain age" scale during the first 4 years of life.