In accordance with the principle of personal autonomy, expert consensus statements on amyotrophic lateral sclerosis (ALS) recommend early engagement with terminal-phase patients on the type of symptomatic treatment to be administered in the event of respiratory failure, since decompensation progresses too rapidly to allow time for a discussion. The French Parliamentary Act on Patients' Rights and End-of-Life Care (dated 22 April 2005) grants individuals the right to refuse unreasonable treatment and obliges physicians to take account of any prior instructions given by a person before he/she became incapable of communicating. The provision of prior instructions is a very reassuring situation for the physician: the autonomous patient indicates his or her choice of end-of-life care. However, there are two pitfalls which must be avoided:(i) holding a discussion for the sole purpose of obtaining prior instructions and (ii) not acknowledging the patient's vulnerability. The present study dealt with 35 ALS patients for whom the question of either intensive care or palliative end-of-life care remained open. Even though the great majority of these individuals were keen to know their exact state of health, 48% refused to consider this circumstance and only 20% expressed prior instructions. These results prompted us to question the ethical dimension of the concept of autonomy beyond its founding formulation: can one envisage an incapacity to confront oneself with the existential question of possible death? In 80% of cases, the physician will have to take a care decision in the absence of any prior instructions from the patient. This amounts to more than respecting a person's autonomy and involves exercising medical responsibility.