The Cruzan case and the Patient Self-Determination Act will encourage patients to specify in advance which life-sustaining treatments they would want if they become mentally incompetent. However, strictly following such advance directives may not always be in a patient's best interests. We sought to determine whether patients differ in how strictly they want advance directives followed.Interview study.Seven outpatient chronic dialysis centers.One hundred fifty mentally competent dialysis patients.Using a structured questionnaire, we asked the subjects whether they would want dialysis continued or stopped if they developed advanced Alzheimer's disease. We then asked how much leeway their physician and surrogate should have to override that advance directive if overriding were in their best interests. Subjects granting leeway were also asked what factors should be considered in making decisions for them.Subjects varied greatly in how much leeway they would give surrogates to override their advance directives: "no leeway" (39%), "a little leeway" (19%), "a lot of leeway" (11%), and "complete leeway" (31%). Subjects also varied in how much they wanted various factors considered in making decisions, such as pain or suffering, quality of life, possibility of a new treatment, indignity caused by continued treatment, financial impact of treatment on family members, and religious beliefs.Strictly following all advance directives may not truly reflect patients' preferences. To improve advance directives, we recommend that physicians explicitly ask patients how strictly they want their advance directives followed and what factors they want considered in making decisions.