CONTEXT: Advance directives or proxy designations are widely recommended, but how they affect doctors' decision making is not well known. OBJECTIVES: The aim of this study was to quantify the influence of advance directives and proxy opinions on doctors' decisions. METHODS: We mailed to all the generalists and internists in French-speaking Switzerland (N = 1962) three vignettes describing difficult decisions involving incapacitated patients. In each case, the advance directive requested that further care be withheld. One vignette tested the impact of a written advance directive vs. a proxy. Another compared the impact of a handwritten directive vs. a formalized document. The third vignette compared the impact of a family member vs. a doctor as a proxy. Each vignette was prepared in three or four versions, including a control version in which no directive or proxy was present. Vignettes were randomly allocated to respondents. We used logistic regression to predict the decision to forgo a medical intervention. RESULTS: Compared with the control condition, the odds of forgoing a medical intervention were increased by the written advance directive (odds ratio [OR] 7.3; P < 0.001), the proxy (OR 7.9; P < 0.001), and the combination of the two (OR 35.7; P < 0.001). The handwritten directive had the same impact (OR 13.3) as the formalized directive (OR 13.8). The effect of proxy opinion was slightly stronger when provided by a doctor (OR 11.3) rather than by family (OR 7.8). CONCLUSION: Advance directives and proxy opinions are equally effective in influencing doctors' decisions, but having both has the strongest effect. The format of the advance directive and the identity of the proxy have little influence on decisions.