Amyotrophic lateral sclerosis (ALS) and adult onset X-linked recessive bulbospinal muscular atrophy (SPMA), constituting the category of adult onset form of motor neuron disease, were analyzed on motor nerve roots. The results of morphometric analysis on ventral spinal roots (VSR) of all spinal segments from ALS and SPMA revealed the following three findings: (1) the large-myelinated alpha-motoneuron fibers were markedly decreased in number throughout all segments; (2) thin-myelinated autonomic preganglionic fibers were almost completely preserved; (3) small-intermediate-myelinated fibers which are considered to correspond to gamma-motoneuron fibers were generally well preserved in ALS, but decreased by one-half to one-third in SPMA. However, all the components of the nerve roots of the oculomotor, trochlear, and abducent nerves were completely preserved in both ALS and SPMA. Moreover, the teased-fiber study showed that the regenerating-sprouting process rarely occurred in the VSR of ALS and SPMA. The present study suggested that the site of the primary lesion seems to be in the alpha-motoneuron fibers in motor neuron diseases, such as ALS or SPMA. However, the marked discrepancy in the pathologic change in the alpha-motoneuron fibers in the VSR and the nerve roots innervating the external ocular muscles was noteworthy.