The purpose of this study was to document the rationale and procedures for withholding and withdrawing life-sustaining treatment in critically ill patients. A prospective observational study was conducted over 12 mo in a Canadian academic intensive care unit. Of the 110 intensive care unit patients who died during the study period, 71 (64.5%) died after treatment was withheld or withdrawn. Compared with the other 39 patients who died despite full therapy, these patients were found to have a longer hospital and ICU stay, more organ systems failed, and a higher rate of malignancy. Intensivists rated poor prognosis for survival and poor quality of life should the patient survive as being the two most important factors when making a decision to withhold or withdraw treatment, while patient age and physical health prior to hospital admission were the two least important factors. There was a consistent approach to withdrawing therapy in 68 of the 71 patients who had treatment either withheld or withdrawn. In these 68 patients, the first step was to write a do-not-resuscitate order, vasopressor drugs were then stopped and, lastly, the patient was weaned from mechanical ventilation and the trachea was extubated. The results of this study demonstrate that life-supporting treatment is commonly withdrawn in critically ill patients when continued therapy is thought to be unlikely to restore the patient to health.