In this paper, we adopt a neurodevelopmental stance to examining frontal variant frontotemporal dementia (fv-FTD) by using experimental procedures from the literature on the growth of social behaviour in children to examine the deficits in social reasoning which may underpin behavioural disturbance in fv-FTD. We present the case of a 47-year-old man with a diagnosis of fv-FTD and severe antisocial behaviour. Tests of general neuropsychology and of executive function were performed. In addition, the patient, JM, was assessed on tasks which test theory of mind. Theory of mind develops in distinct stages through childhood and is a core ability to represent the thoughts and feelings of others, independent of the level of intellectual ability. The results indicate relatively intact general neuropsychological and executive function, but extremely poor performance on tasks of theory of mind. This indicates a dissociation of social cognition and executive function suggesting that in psychiatric presentations of fv-FTD there may be a fundamental deficit in theory of mind independent of the level of executive function. The implications of this finding for diagnostic procedures and possible behavioural management are discussed.