How many diagnostic errors occur? How often do errors cause harm, and how serious is that harm? Do we understand the major causes of diagnostic errors? Really, we don't know how to answer these questions! This article seeks to define a challenge facing all healthcare risk managers, whose usual methods of identifying and analyzing errors have not, and cannot, supply this missing information. What should risk managers do about diagnostic error? Our medical literature acknowledges the existence of a problem, but offers few practical solutions. This article will review some promising theories from the literature regarding how to identify and remediate diagnostic errors, and identify some tools and resources available to risk managers.