The accuracy of proxies when they interpret advance directives or apply substituted decision-making criteria has been called into question. It therefore became important to know if the Andalusian Advance Directive Form (AADF) can help to increase the accuracy of proxies' predictions. The aim of this research was to compare the effect of the AADF on the accuracy of proxies' predictions about patients' preferences with that gained from informative and deliberative sessions about end-of-life decision making. A total of 171 pairs of patients and their proxies were randomized to three groups. The control group's answers to the Life Sustaining Preferences Questionnaire (LSPQ) were compared with their proxies' answers to the same questionnaire. In one intervention group, the patients had already completed the AADF and given it to their proxies, who used it to guide their own answers to the LSPQ. In the second intervention (discussion) group, both patients and proxies attended two educative sessions guided by trained nurses and later filled in the LSPQ. Comparisons of accuracy and other variables showed a strong association with the discussion group. The findings show that promoting communication between patients and their proxies improves the accuracy of proxies' predictions much more than isolated use of the AADF form.