The complexities associated with advance treatment decisions may result in decisional uncertainty and poorly informed choices, particularly among vulnerable populations such as minorities and patients with limited health literacy. Using a hypothetical scenario, we assessed uncertainty about treatment preferences among 205 chronically ill, English/Spanish-speaking older adults from a county outpatient clinic in San Francisco. Participants were read a scenrio that described a very poor health state and poor outcome of life support treatment. Participants were asked to imagine that they were in this scenario and to choose either: all life support treatments; try life support with an option of stopping; or no life support. They were then asked how certain they were about this decision. Forty-five percent of participants were uncertain about their decision. In adjusted multivariate analysis, Latinos and Asian/Pacific Islanders were more likely than Whites to be uncertain about their treatment decisions, as were participants with limited versus adequate literacy and poor versus good self-rated health. Many patients may be uncertain about advance treatment preferences. Culturally sensitive, literacy-appropriate tools are needed to address decisional uncertainty and to help patients prepare for decision making about their future health care.