The application of computing to health care, and particularly to electronic patient records, offers major benefits but raises issues of confidentiality and of potential misuse. Sound access control mechanisms are therefore important, but most models focus upon informed consent by the data subject. This raises challenges in mental health care, and for other vulnerable patients including those comatose, and the severely ill and temporarily distressed. Published algorithms which are used to control record access within a controlled environment therefore have value, as a means of ensuring an open and informed, yet ethically sound, solution. The paper describes the background and issues, and gives an example of such an algorithm.