A small number of patients have recently been described with a sporadic neurodegenerative disease, associated with the neuropathological finding of neurofilament-immunoreactive neuronal inclusions. The clinical and pathological spectrum of this new disease entity has yet to be fully defined. We describe an additional case of "neurofilament inclusion body disease" (NIBD) with several unusual features. This young woman, who suffered from rapidly progressive frontotemporal dementia (FTD) and features of primary lateral sclerosis (PLS), died at age 29. Neuropathological examination disclosed numerous neuronal cytoplasmic inclusions in many regions of the central nervous system. The inclusions varied in morphology with some being immunoreactive for ubiquitin while others showed strong positivity for neurofilament proteins. Intranuclear neuronal inclusions were also present. There was no significant tau or alpha-synuclein pathology. There was severe degeneration of the corticospinal tracts but lower motor neurons were normal in number and morphology. This case confirms that NIBD should be considered in the differential diagnosis of FTD, particularly in young patients. In addition, it extends the clinical phenotype of NIBD to include PLS and better defines the anatomical distribution and morphology of the pathological lesions.