Different clinical criteria for diagnosing dementia were compared in a sample of 69 patients with motor neurone disease (MND). Participants' performances on a computerised battery of neuropsychological tests were evaluated to assess the usefulness of these tests in predicting dementia in MND. The results indicated that when diagnostic criteria for frontotemporal (FTD) were used as part of a questionnaire method of diagnosing dementia the incidence of dementia in MND was considerably greater than traditional estimates suggest. Through a series of logistic and multiple regressions the results demonstrated that neuropsychological test performance related well to diagnostic classifications of dementia. MND patients with a clinical diagnosis of dementia were likely to demonstrate impaired new learning; poor working memory and planning; slowness in information processing and rigidity in thinking. These features, which are typical of cases of FTD, suggest that the dementia of MND is usefully characterised as a form of FTD. The finding that neuropsychological impairment correlated with behavioural features of dysexecutive impairment in daily living, indicates that the management focus in MND must be broadened to include cognitive/behavioural issues.