Responsibility for the protection of human research subjects is shared by investigators, research ethics committees, sponsors/funders, research institutions, governments and, the focus of this article, physicians who enrol patients in clinical trials. The article describes the general principles of the patient-physician relationship that should regulate the participation of physicians in clinical trials and proposes guidelines for determining when and how such participation should proceed. The guidelines deal with the following stages of the trial: when first considering participation, when deciding whether to enrol patients, when asking patients to participate, when the trial is underway and when it is completed.