This study presents a structural and clinical description of a patient with dementia of motor neuron disease (D-MND), and compares and contrasts the neuropsychological characteristics of this patient with those of a group of patients with Alzheimer's disease (AD) matched by severity of dementia. The D-MND patient as well as the AD patients performed abnormally on all tasks that assessed executive/frontal functions. However, the D-MND patient tended to be more impaired than AD patients on tasks that required high speed decisions and on shifting from one strategy to another. Deficits in memory, language, and lexical-semantic abilities were also apparent in this patient. The co-occurrence of signs and symptoms characteristics of 'subcortical' dementia with those of 'cortical' dementia suggest that the executive system dysfunction may be secondary to subcortical pathology.