After completion of neuronal migration to form the cerebral cortex, axons undergo rapid elongation to their intra- and subcortical targets, from midgestation through infancy. We define axonal development in the human parietal white matter in this critical period. Immunocytochemistry and Western blot analysis were performed on 46 normative cases from 20-183 postconceptional (PC) weeks. Anti-SMI 312, a pan-marker of neurofilaments, stained axons as early as 23 weeks. Anti-SMI 32, a marker for nonphosphorylated neurofilament high molecular weight (NFH), primarily stained neuronal cell bodies (cortical, subcortical, and Cajal-Retzius). Anti-SMI 31, which stains phosphorylated NFH, was used as a marker of axonal maturity, and showed relatively low levels of staining (approximately one-fourth of adult levels) from 24-34 PC weeks. GAP-43, a marker of axonal growth and elongation, showed high levels of expression in the white matter from 21-64 PC weeks and lower, adult-like levels beyond 17 postnatal months. The onset of myelination, as seen by myelin basic protein expression, was approximately 54 weeks, with progression to "adult-like" staining by 72-92 PC weeks. This study provides major insight into axonal maturation during a critical period of growth, over an age range not previously examined and one coinciding with the peak period of periventricular leukomalacia (PVL), the major disorder underlying cerebral palsy in premature infants. These data suggest that immature axons are susceptible to damage in PVL and that the timing of axonal maturation must be considered toward establishing its pathology relative to the oligodendrocyte/myelin/axonal unit.