The histological and ultrastructural features of axonal swellings are described in a cerebral biopsy specimen from a 6-year-old girl with infantile neuroaxonal dystrophy. In agreement with previous reports, several swellings were identified as axonal terminals, and it is postulated that the prevailing axonal swellings in cerebral cortex represent dystrophic boutons. Microscopically, dystrophic cortical boutons are morphologically different from typical subcortical spheroids and can be easily identified in routine histological preparations. Five ultrastructural elements were present in most axonal swellings: clusters of characteristic membranous bodies, stacks of elongated membranes, mitochondria, groups of vesicles, and an amorphous matrix. In spite of widespread axonal enlargement, the cerebral cortex was not thickened, and, in fact, the gyri looked atrophic in computerized axial tomograms. These findings suggest that some normal cortical element must be deficient, but such a structure remains to be identified.