We review the practical importance of lobar atrophy in frontotemporal dementia (FTD), for diagnosis and prognosis. We discuss specific patterns of frontotemporal atrophy that denote clinical and pathological subtypes of FTD (e.g. semantic dementia). We also discuss the unsatisfactory clinical experience of interpreting MRI scans in individual FTD cases, especially the behavioural presentations (without aphasic or motor impairments). This issue is explored by examining the FTD phenocopy concept. Lobar atrophy emerges as a key observation in defining behavioural FTD patients whose symptoms are likely to progress. In a situation where objective clinical data are few, we highlight the importance of applying caution before diagnosing FTD is the absence of visible brain atrophy.