One of the main goals of end-of-life care is to achieve the best quality of life (QOL) for patients and their families. Quality of life, therefore, represents a significant outcome indicator to evaluate end-of-life care interventions. However, nonresponse bias and nonrandom missing data in QOL research at the end-of-life limits the generalizability and threatens the internal validity of the study findings. The use of family proxy of patients' QOL has been suggested as a solution. Demonstration of satisfactory levels of agreement between proxies and patients is warranted before family caregivers' or other proxies' assessments can be employed when patients cannot provide their own information. Contrary to the conclusion made by Sprangers and Aaronson [The Role of Health Care Providers and Significant Others in Evaluating the Quality of Life of Patients with Chronic Disease: A Review. J. Clin. Epidemiol. 1992, 45, 743-760], it is suggested from this review of literature that terminal cancer patients and their family caregivers agreed at least moderately well on the patients' QOL. The bias introduced by the use of family informants is generally of a modest magnitude. When discrepancies existed, without exception, family caregivers held a more negative view of patients' QOL than did patients. When using family proxies, this is important to remember. The degree of agreement between terminal cancer patients' and their family caregivers' assessments varies as a function of the dimensions of QOL being measured and the patient's health status. However, the accuracy of family caregivers' assessments can be improved by assessing both patients and family caregivers concurrently over time. Several suggestions for future research are provided to better understand the influencing factors of agreement between patients and family assessments and to enhance the quality of statistical analyses on this topic.