Background: Advance directives are intended to extend patient autonomy into periods of mental incompetence. However, for advance directives to fulfill this objective, patients' choices must be reasonably consistent over time. Thus, we assessed the stability of the advance treatment decisions of patients and members of the public. Methods: In a prospective cohort study of 495 outpatients and 102 members of the public, we studied the stability of scenario- and treatment-specific choices. Subjects completed an advance directive, which included four illness scenarios with 11 treatment choices in each, as part of a questionnaire. A second interview was completed by 296 patients and 78 members of the public after 6 to 12 months; 154 patients completed a third interview after a further 6 to 12 months. We assessed stability by comparing each choice between interviews. Results: Stability of choices was moderately high among patients (pooled [kappa]=0.39) and members of the public (pooled [kappa]=0.48). Stability improved with repeat interview (pooled [kappa]=0.47 among patients). Patients who had discussions with their physicians showed more improvement ([kappa]=0.57) than others. Patients had a wide range of personal stability levels 0% to 100%), but individuals starting out stable rarely became less so (93% of the patients with 85% to 100% stability maintained this level of stability on the third interview). Hospitalized patients showed no significant difference in stability at the second interview, but their stability was not improved at the third interview. Conclusions: Our findings generally support the use of advance directives. Most people made moderately stable decisions when using scenario- and treatment-specific directives, and stability improved after they reviewed the decisions, especially among those who had discussions with their physicians. Recent hospitalization did not decrease stability, although it appeared to reduce the improvement that others achieved with repeat interview. These findings suggest that advance directives can be relied on 1 to 2 years after completion to reflect a patient's choices.