There are a number of circumstances under which it may be argued that compulsory blood testing is justified. In the face of demands for the right to know, how does a hospital or other health care provider ensure that a patient's right to privacy is respected? Which right should prevail: the right to know or the right to privacy? This article reviews Canadian common law and Alberta legislation affecting blood testing for communicable diseases, considers the effect of the Canadian Charter of Rights and Freedoms (the charter) on legislation which imposes non-consensual blood testing and concludes that compulsory blood testing based upon reasonable and individualized suspicion of infection or demonstrated necessity may be upheld under the charter if it can be established that the results of such testing will be of significant use. However, in the majority of circumstances, it will be difficult to establish that non-consensual blood testing of patients is reasonably justified in a free and democratic society under the charter. Moreover, even if a health care provider could legally implement a mandatory blood testing policy, there are a number of good reasons not to do so.