A quantitative study was performed on spinal cord lesions in seven patients with X-linked recessive spinal and bulbar muscular atrophy. The myelinated fiber density of the lateral corticospinal tracts at the T7 cord level was well preserved for both large and small myelinated fibers. On the other hand, neurons in the L4 ventral horn were markedly depleted; marked loss was noted of the large alpha and medium-sized gamma motor neurons located in the lateral and medial nuclei as well as the small neurons in the intermediate zones of the ventral horn. These results suggest that myelinated fiber density and fiber-size distribution in the corticospinal tract are well preserved and that neuronal loss in the ventral horns is not restricted to alpha and gamma motoneurons but also involves small interneurons.