Objectives Apathy is one of the most common and disabling syndromes of dementia. Clinical apathy expression and neuroanatomical basis of apathy seem to differ between behavioral variant frontotemporal dementia (bvFTD) and Alzheimer's disease (AD), although evidence is scarce and poorly understood. Our main purposes were to compare the clinical apathy profile from patients with bvFTD and AD and analyze the relationship between apathy and brain metabolism measured using positron emission tomography imaging with 18F fluorodeoxyglucose (FDG‐PET). Methods Forty‐two bvFTD, 42 AD, and 30 healthy volunteers without cognitive or behavioral complaints were included. Apathy was defined using Robert's 2009 diagnostic criteria, and specific apathy characteristics were assessed with the Lille Apathy Rating Scale. All participants underwent FDG‐PET brain scan to provide data for voxel‐based morphometric analysis. Results Multivariate analysis showed that subjects affected by bvFTD displayed greater impairment of emotional apathy and self‐awareness in comparison with AD sample. Additionally, FDG‐PET imaging analyses revealed that apathy was associated with different neuroanatomical substrates in each dementia group: left lateral prefrontal, medial frontal/anterior cingulate, lateral orbitofrontal and anterior insular cortices in bvFTD, and right anterior cingulate in AD. Conclusions These results support that apathy is a complex syndrome, with different clinical expressions across different pathological conditions. Those differences in qualitative aspects of apathy seem to be associated with differences in the damage sites, as shown by our FDG‐PET imaging analysis. Our findings provide a better knowledge about pathophysiology of apathy in dementia, which could have practical implications for therapeutic management. Copyright © 2017 John Wiley & Sons, Ltd.