Cognitive impairment in the absence of lesions indicative of Alzheimer's disease and other dementing conditions has long been recognized in a subgroup of patients with motor neuron disease MND), including amyotrophic lateral sclerosis. However, the mechanisms underlying this cognitive deterioration and its relationship with the relatively selective involvement of motor neurons remains elusive. We used histo- and immunocytochemical labeling methods to study the nitrogen monoxide (NO; a.k.a. nitric oxide) synthase (NOS)-/NADPH diaphorase-containing neurons (NOSN) in three patients with MND and dementia (MND+D), two patients with MND without dementia, and 19 controls that included patients with Alzheimer and non-Alzheimer dementias. Patients with MND+D, but not those with MND without dementia, exhibit numerous dystrophic perikarya and neurites throughout all sensory, motor, association, and limbic neocortices examined. Interestingly, affected NOSN appear to correspond to some subtypes (smooth stellate and spiny neurons), while other neurons containing the same molecular phenotype (such as layer I local circuit neurons and layer II granule cells) are either spared or significantly less affected. These observations indicate that cognitive impairment and dementia in MND may be due, at least in part, to a pancortical involvement of certain types of NOSN. Consequently, the elucidation of the factors that make NOSN vulnerable in MND, and the prevention or pharmacological palliation of their loss, may eventually help to prevent or ameliorate cognitive impairment in MND and may also shed some light on the nature of the insult that targets motor neurons.