In vivo magnetic resonance (MR) images of the brain in neonates and infants up to 24 months of age were retrospectively studied to evaluate normal maturation patterns in the brain. Sequential changes in myelination of various brain structures and development of the corpus callosum were noted. At birth and for the first 4-6 months of life, signal intensities of gray and white matter are the reverse of those seen in an adult brain, with the signal intensity of white matter being lower than that of gray matter on T1-weighted images and higher than that of gray matter on T2-weighted images. With advancing age, white matter shows a progressive increase in signal intensity on T1-weighted images. On T2-weighted images, high-signal-intensity unmyelinated white matter progressively changes to myelinated white matter of a signal intensity lower than that of gray matter. At birth, the corpus callosum is isointense relative to white matter and progressively increases in signal intensity, so that at age 8 months the corpus callosum has an appearance identical to that of an adult. Familiarity with the temporal sequence of normal myelination as seen with MR imaging is helpful in the diagnosis of pathologic processes involving white matter.