Bioethicists have articulated an ideal of shared decision making between physician and patient, but in doing so the role of clinical uncertainty has not been adequately confronted. In the face of uncertainty about the patient's prognosis and the best course of treatment, many physicians revert to a model of nondisclosure and nondiscussion, thus closing off opportunities for shared decision making. Empirical studies suggest that physicians find it more difficult to adhere to norms of disclosure in situations where there is substantial uncertainty. They may be concerned that acknowledging their own uncertainty will undermine patient trust and create additional confusion and anxiety for the patient. We argue, in contrast, that effective disclosure will protect patient trust in the long run and that patients can manage information about uncertainty. In situations where there is substantial uncertainty, extra vigilance is required to ensure that patients are given the tools and information they need to participate in cooperative decision making about their care.