Three years after the implantation of electrodes in the subthalamic nucleus (STN) and the start of deep brain stimulation (DBS) for advanced Parkinson's disease, a 62-year-old man was admitted because of a stimulation-related manic state that did not respond to treatment with a mood stabiliser and that led to chaotic behaviour, megalomania, serious financial debts and mental incompetence. Although adjustment of the stimulation parameters resulted in a normophoric state with a return of insight and capacity to judge, this was only at the cost of a serious exacerbation of his motor symptoms that left the patient bedridden. There was no therapeutic margin between the two states. Ultimately, there seemed to be only two alternatives: to admit the patient to a nursing home because of serious invalidity, but mentally in good condition, or to admit the patient to a chronic psychiatric ward because of a manic state, but with acceptable motor capacity and ADL functions. Thorough ethical evaluation followed. When not being stimulated, the patient was considered competent to decide about his own treatment; in this condition the patient chose for the second option. In accordance with his own wishes he was therefore legally committed to a chronic ward in the regional psychiatric hospital. The current ethical views on mental competence do not consider the potential influence of modern methods of treatment such as STN-DBS on this capacity.