Ubiquitin-immunoreactive (ub-ir) neuronal cytoplasmic inclusions are characteristically found in the extramotor cortex in patients with motor neuron disease and dementia (MND-dementia) and a subset of patients with frontotemporal dementia without motor symptoms (FTD-MND type). Recently, ub-ir neuronal intranuclear inclusions have been described in a small number of patients with familial FTD-MND type. To better define the sensitivity and specificity of this pathological change, we examined postmortem tissue from 14 patients with FTD-MND type (8 familial, 6 sporadic), 10 cases of MND-dementia (5 familial, 5 sporadic), and 19 cases of MND with no history of cognitive dysfunction (2 familial, 17 sporadic). Numerous intranuclear inclusions were found in multiple anatomic sites in 6/8 cases of familial FTD-MND. Rare intranuclear inclusions were present in the hippocampal dentate granule cells in 1 case of familial MND-dementia. No sporadic cases had intranuclear inclusions. These findings suggest that intranuclear inclusions are specific for familial FTD and may identify a subset of families with a common molecular pathogenesis. Although intranuclear inclusions are most characteristic of families in which the clinical presentation is pure FTD, they may also be found in some pedigrees with both FTD and MND; further supporting the hypothesis that FTD-MND type and MND-dementia represent a clinicopathological spectrum of disease.