Involvement of Onuf's nucleus (ON) in 28 cases of amyotrophic lateral sclerosis/motor neuron disease (MND/ALS) with different clinical syndromes is reported. Although significant neuronal loss was absent, all cytoskeletal abnormalities typical of alpha-motor neurons in MND/ALS were found in ON. Spheroids were detected in 53.5% of cases; 0.6-4.5% of ON neurons contained Bunina bodies, which were present in 42.8% of cases. Ubiquitin-reactive inclusions (UBRI) of filamentous and hyaline type were found in 57.1% of cases and in 1.2-10.7% of ON neurons. Cases with pyramidal tract involvement (ALS) were involved by UBRI in 76.5%, whereas cases with progressive spinal muscular atrophy revealed the same inclusions in only 27.2%. No similar inclusions were present in sacral parasympathetic intermediolateral nucleus. It can be concluded, therefore, that ON belongs to the somatic motor system and is principally vulnerable to MND/ALS, albeit to a lower degree.