Making decisions on the basis of evidence is a central tenet of all health-care disciplines, including public health. However, it is not entirely clear what it means to base decisions on evidence; debates on evidence-based approaches often lack a clear understanding of the nature of evidence and obscure the normative underpinnings of evidence. Public health decision making requires an acceptance of limitations such as the availability of funding for research to provide complete evidence for any given decision, the ethical constraints on the creation of certain types of evidence and the ongoing dilemma between the need to take action and the need to gather more information. Using the example of the SARS outbreak in Canada, the inter-relationships between evidence and ethics are explored. I outline a set of critical questions for the global public health community to discuss regarding the nature of the relationship between evidence-based public health practice and ethics.