Morphometric studies of the 5th lumbar ventral spinal roots and hypoglossal nerves were made in twenty-one patients with motor neuron diseases and 12 age-matched controls. To evaluate the correlation between the numbers of the myelinated fibers and the patterns of white matter degeneration in spinal cords, the latter were classified into three groups: group A with selectively demarcated degeneration of lateral and anterior corticospinal tracts, group B with diffuse degeneration of antero-lateral spinal columns and group C with no degeneration of the spinal white matter. Although the numbers of large myelinated fibers of the 5th lumbar ventral roots were decreased in all three groups, the numbers in group C were significantly better preserved than those in group A. Even in the hypoglossal nerve, the numbers of total and large myelinated fibers were decreased and in addition, the longer the duration of bulbar signs and the more severe the pathological changes found in the hypoglossal nuclei, the fewer the number of myelinated fibers. Especially group A showed a significantly more severe decrease in the numbers of total and large myelinated fibers of the 5th lumbar ventral roots and hypoglossal nerves than group C. These findings suggest that the degeneration of the upper motor neuron accelerates that of the lower motor neuron system in motor neuron diseases.