Fourteen cases of amyotrophic lateral sclerosis (ALS) were investigated neuropathologically, emphazising the sacral spinal cord which contains Onuf's nucleus X. The nucleus innervates the pelvic sphincters. In two cases, small striated pelvic muscles were studied. No changes characteristic of ALS were observed in Onuf's nucleus X, not even in 8 cases in which other caudal motoneuron nuclei presented a severe loss of neurons. The striated sphincters proper demonstrated no signs of neurogenic atrophy in contrast to muscles in the limbs. The bulbo- and ischiocavernosus muscles, also supposedly innervated by Onuf's nucleus, were without pathological changes. Moreover, the latter two muscles were found to have a composition very similar to that of the sphincters. This indicates that a characteristic morphology of the nucleus X-innervated muscles exists. A review of the clinical records of all the cases revealed, that although 8 demonstrated severe involvement of the lower extremities, only one presented vesico-rectal dysfunction which could be ascribed to ALS. But even in this case, the pelvic closure mechanisms appeared to be intact. The preservation of continence in ALS is related to the neuropathological findings, and the observations are compared with previous neuropathological studies concerning Onuf's nucleus X as well with experimental studies including this nucleus. It is pointed out that structural and biochemical differences must exist between nucleus X neurons and other motoneurons.