Betrayal trauma, a type of psychological trauma that occurs when the people or institutions a person depends on for survival violate that person’s trust (Freyd, 2008), has been shown to further intensify the negative effects of trauma (Martin, Cromer, DePrince, & Freyd, 2013). A recent extension of this concept, which can cause additional psychological harm to victims, is institutional betrayal (IB), an institutional failure to respond supportively to a traumatic event (Smith & Freyd, 2013). The medical system is an important, but as of yet unstudied institution with regard to betrayal. The frequency of medical errors, ineffectiveness of their disclosure, and the lack of clarity in clinical informed consent may give rise to feelings of betrayal. The current research aimed to develop a psychometrically sound questionnaire that assesses patients’ perceptions of IB. A qualitative study completed by 14 participants investigated the kind of experiences that lead to feelings of IB in the medical system. Content analysis was used to analyze the data, and the overall themes of doctor and system level betrayal guided the development of the IB questionnaire. A quantitative study completed by 352 participants tested the reliability and validity of the measure. An exploratory factor analysis and reliability statistics found three stable and reliable factors of IB: negative healthcare experiences, cognitive-affective reactions, and systemic responses to these experiences. Subsequent correlations and hierarchical multiple regressions found evidence supporting the validity of the IB questionnaire. As expected, IB predicted poorer mental health functioning even after controlling for demographics, and previous traumatic experiences.