Proponents of evidence-based medicine (EBM) suggest that a hierarchy of evidence is needed to guide medical research and practice. Given a variety of possible evidence hierarchies, however, the particular version offered by EBM needs to be justified. This article argues that two familiar justifications offered for the EBM hierarchy of evidence—that the hierarchy provides special access to causes, and that evidence derived from research methods ranked higher on the hierarchy is less biased than evidence ranked lower—both fail, and that this indicates that we are not epistemically justified in using the EBM hierarchy of evidence as a guide to medical research and practice. Following this critique, the article considers the extent to which biases influence medical research and whether meta-analyses might rescue research from the influence of bias. The article concludes with a discussion of the nature and role of biases in medical research and suggests that medical researchers should pay closer attention to social mechanisms for managing pervasive biases.