Ensuring confidentiality is the cornerstone of trust within the doctor–patient relationship. However, health care providers have an obligation to serve not only their patient’s interests but also those of potential victims and society, resulting in circumstances where confidentiality must be breached. This article describes the attitudes of mental health professionals (MHPs) when patients disclose past crimes unknown to the justice system. Twenty-four MHPs working in Swiss prisons were interviewed. They shared their experiences concerning confidentiality practices and attitudes towards breaching confidentiality in prison. Qualitative analysis revealed that MHPs study different factors before deciding whether a past crime should be disclosed, including: (1) the type of therapy the prisoner-patient was seeking (i.e., whether it was court-ordered or voluntary), (2) the type of crime that is revealed (e.g., a serious crime, a crime of a similar nature to the original crime, or a minor crime), and (3) the danger posed by the prisoner-patient. Based on this study’s findings, risk assessment of dangerousness was one of the most important factors determining disclosures of past crimes, taking into consideration both the type of therapy and the crime involved. Attitudes of MHPs varied with regard to confidentiality rules and when to breach confidentiality, and there was thus a lack of consensus as to when and whether past crimes should be reported. Hence, legal and ethical requirements concerning confidentiality breaches must be made clear and known to physicians in order to guide them with difficult cases.