Our case demonstrates that identifying the underlying C9orf72 dementia can be complicated by the fact that results of neuropsychological testing and imaging studies can be more suggestive of Alzheimer’s disease (pronounced hippocampal atrophy, biparietal and temporomesial hypometabolism, and early deficits in episodic verbal memory and visuospatial abilities) than of frontotemporal dementia. Thus, while frontal hypoperfusion or atrophy with relative sparing of the temporal lobes has been described as a predominant pattern in several patients with C9orf72 dementia (7, 8), neurodegenerative dysfunction might be much more variable in others: C9orf72 dementia might present as “mixed dementia” with features of both frontotemporal dementia and Alzheimer’s disease. This explains why C9orf72 dementia can easily be misdiagnosed as Alzheimer’s disease (4).