Neuroscientific research into mental health commands generous funding, suggesting neuroscience is understood by a variety of actors and institutions as having significant potential to enhance the therapeutic practices of psychiatrists. This article interrogates this 'therapeutic promise' of neuroscience through the case study of the psychiatric condition personality disorder. Specifically, the focus is on the promissory discourse of clinicians specialising in the management of two variants of personality disorder--antisocial personality disorder and psychopathy--and researchers investigating the neurobiology of these constructs. The article discusses the respondents' ambivalent expectations regarding the therapeutic promise of brain research, and shows how these are structured by understandings of the ontology of personality disorder. In turn, these ambivalences direct our attention to practical issues surrounding the potential of neuroscience to translate into and enhance clinical practice, as well as theoretical concerns revolving around the place and role of the biological within contemporary neuroscience, psychiatry and psychology. In sum, the necessity of large material and symbolic investments in neuroscience should, perhaps, be reflected upon more critically, and analytic encounters with this discipline must keep in mind it's at times surprising commitment to the realms of the social and the psychological.