The olfactory bulb with its unique architecture was studied for neuronal maturation in human fetuses. Neuroblasts stream into the olfactory bulb from the rostral telencephalon and secondarily migrate radially. The transitory olfactory ventricular recess regresses postnatally. Olfactory is the only sensory system without thalamic projections but incorporates intrinsic thalamic equivalents. The bulb is a repository of progenitor cells. Maturation of the bulb and tract was studied in 18 normal human fetuses of 16-41 weeks gestation; mid-gestational twins with hydrocephalus; 7 arrhinencephaly/holoprosencephaly; 2 olfactory dysgeneses. Multiple immunoreactivities were performed. Synaptophysin around mitral neurons, in a few synaptic glomeruli and concentric lamination of the outer granular layer, was seen at 16 weeks. Outer granular neurons exhibited NeuN at 16 weeks, only 2/3 were reactive at term. Concentric alternating sheets of granular neurons and their dendrodendritic synapses are seen during maturation. Calretinin reactivity is seen in neurons and neurites, primary olfactory nerve axons, periglomerular cells and neuroepithelial cells surrounding the ventricular recess; reactivity occurs later in synaptic glomeruli than with synaptophysin; not all glomeruli are strongly reactive even at term. Nestin- and vimentin-reactive bipolar progenitor cells were demonstrated at all ages and extend into the olfactory tract. Myelin is demonstrated by Luxol fast blue (LFB) only postnatally. In hydrocephalus, the olfactory recess is dilated. Mitral cell dispersion, disrupted glomeruli, heterotopia and maturational delay are seen in some dysgeneses. Malformations exhibit unique findings. Fusion of hypoplastic bulbs can occur. Abnormal architecture is seen in hemimegalencephaly. More documentation of olfactory dysgenesis is needed in other major brain malformations.