As the population ages, more adults will develop impaired decision-making capacity and have no family members or friends available to make medical decisions on their behalf. In such situations, a professional guardian is often appointed by the court. This official has no preexisting relationship with the impaired individual but is paid to serve as a surrogate decision maker. When a professional guardian is faced with decisions concerning life-sustaining treatment, substituted judgment may be impossible, and reports have repeatedly suggested that guardians are reluctant to make the decision to limit care. Physicians are well positioned to assist guardians with these decisions and safeguard the rights of the vulnerable persons they represent. Doing so effectively requires knowledge of the laws governing end-of-life decisions by guardians. However, physicians are often uncertain about whether guardians are empowered to withhold treatment and when their decisions require judicial review. To address this issue, we analyzed state guardianship statutes and reviewed recent legal cases to characterize the authority of a guardian over choices about end-of-life treatment. We found that most state guardianship statutes have no language about end-of-life decisions. We identified 5 legal cases during the past decade that addressed a guardian's authority over these decisions, and only 1 case provided a broad framework applicable to clinical practice. Work to improve end-of-life decision making by guardians may benefit from a multidisciplinary effort to develop comprehensive standards to guide clinicians and guardians when treatment decisions need to be made.