In adult mammals, the subventricular zone is known to be one of the few sources for new neurons in the brain, but the contributions of these progenitors to distinct regions of the postnatal human brain are poorly understood. A study of human brain specimens from neurosurgical resections and autopsies ranging in age from birth to 84 years reveals extensive migration and proliferation of neuronal precursors along the rostral stream in human infants. After about 18 months of age, this subsides and is almost extinct in adulthood. During this limited time of neurogenesis, a novel stream of cells destined for the prefrontal cortex was also observed. These findings reconcile some conflicting reports in the literature and reveal new trajectories for tangentially migrating immature neurons during human brain development. The subventricular zone of many adult non-human mammals generates large numbers of new neurons destined for the olfactory bulb1,2,3,4,5,6. Along the walls of the lateral ventricles, immature neuronal progeny migrate in tangentially oriented chains that coalesce into a rostral migratory stream (RMS) connecting the subventricular zone to the olfactory bulb. The adult human subventricular zone, in contrast, contains a hypocellular gap layer separating the ependymal lining from a periventricular ribbon of astrocytes7. Some of these subventricular zone astrocytes can function as neural stem cells in vitro, but their function in vivo remains controversial. An initial report found few subventricular zone proliferating cells and rare migrating immature neurons in the RMS of adult humans7. In contrast, a subsequent study indicated robust proliferation and migration in the human subventricular zone and RMS8,9. Here we find that the infant human subventricular zone and RMS contain an extensive corridor of migrating immature neurons before 18 months of age but, contrary to previous reports8, this germinal activity subsides in older children and is nearly extinct by adulthood. Surprisingly, during this limited window of neurogenesis, not all new neurons in the human subventricular zone are destined for the olfactory bulb—we describe a major migratory pathway that targets the prefrontal cortex in humans. Together, these findings reveal robust streams of tangentially migrating immature neurons in human early postnatal subventricular zone and cortex. These pathways represent potential targets of neurological injuries affecting neonates.