Olfactory mucosa was harvested by intranasal biopsy from a man with Kallmann's syndrome in whom the absence of the olfactory bulbs was documented by magnetic resonance imaging. On electron microscopic examination, several pathologic changes were evident in the olfactory mucosa. First, most olfactory neurons lacked cilia (ie, were morphologically immature). Second, the fila olfactoria had fewer than the normal number of axons, and a large proportion of them were apparently undergoing electron lucent degeneration. Finally, neuromatous collections of axons were seen superficial to the basement membrane in the epithelium. Similar changes have been observed in the mucosa of experimentally bulbectomized rodents. Accordingly, a constellation of pathologic changes--axonal degeneration, neuronal immaturity, and the formation of intraepithelial neuromas--seems to be characteristic of olfactory mucosa that cannot innervate the olfactory bulb in both humans and animals. On the basis of our observations, it is worth investigating the status of the olfactory bulb in other forms of human anosmia in which similar morphological changes are observed in the mucosa, such as persistent posttraumatic anosmia and isolated congenital anosmia.