Decisions and determinants of decisions to prolong or shorten life in the course of fatal diseases like ALS are poorly understood. Decisions and desire for hastened death of N = 93 ALS patients were investigated in a prospective longitudinal approach three times in the course of 1 year. Determinants of decisions were evaluated: quality of life (QoL), depression, feeling of being a burden, physical function, social support and cognitive status. More than half of patients had a positive attitude towards life-sustaining treatments and they had a low desire for hastened death. Of those with undecided or negative attitude, 10 % changed attitudes towards life-sustaining treatments in the course of 1 year. Patients' desire to hasten death was low and decreased significantly within 1 year despite physical function decline. Those with a high desire for hastened death decided against invasive therapeutic treatments. QoL, depression and social support were not predictors for vital decisions and remained stable. Feeling of being a burden was a predictor for decisions against life-supporting treatments. Throughout physical function loss, decisions to prolong life are flexibly adapted while desire to shorten life declines. QoL was stable and not a predictor for vital decisions, even though anticipated low QoL has been reported to be the reason to request euthanasia. In contrast, feeling of being a burden in decision making needs more attention in clinical counselling. Considering a patient's possible adaptation processes in the course of a fatal disease is necessary.