In the adult brain, progenitor cells remaining in the subventricular zone (SVZ) are frequently identified as glial fibrillary acidic protein (GFAP)-positive cells that retain attributes reminiscent of radial glia. Because the very high expression of monoamine oxidase B (MAO-B) in the subventricular area has been related to epithelial and astroglial expression, we sought to ascertain whether it was also expressed by progenitor cells of human control and Alzheimer's disease (AD) patients. In the SVZ, epithelial cells and astrocyte-like cells presented rich MAO-B activity and immunolabeling. Nestin-positive cells were found in the same area, showing a radial glia-like morphology. When coimmunostaining and confocal microscopy were performed, most nestin-positive cells showed MAO-B activity and labeling. The increased progenitor activity in SVZ proposed for AD patients was confirmed by the positive correlation between the SVZ nestin/MAO-B ratio and the progression of the disease. Nestin/GFAP-positive cells, devoid of MAO-B, can represent a distinct subpopulation of an earlier phase of maturation. This would indicate that MAO-B expression takes place in a further step of nestin/GFAP-positive cell differentiation. In the early AD stages, the discrete MAO-B reduction, different from the severe GFAP decrease, would reflect the capacity of this population of MAO-B-positive progenitor cells to adapt to the neurodegenerative process.