Deep brain stimulation (DBS) has in some cases been associated with significant psychological effects and/or personality change. These effects occur sometimes as acute changes experienced intraoperatively or during the initial setting of the stimulator and sometimes as longer term progressive changes in the months following surgery. Sometimes they are the intended outcome of treatment, and in other cases they are an unintended side-effect. In all of these circumstances some patients and caregivers have described the psychological effects of DBS as frightening (...) or disconcerting. I trace the source of these negative reactions to the fear that stimulation-related psychological and personality changes represent a threat to personal identity and agency. This issue has implications both for philosophical theories of personal identity and agency and for clinical concerns. A narrative account of personal identity is developed to illuminate the nature of the threat to identity and agency DBS potentially poses, and to suggest steps that might be taken to mitigate and avoid these threats. (shrink)