Confidentiality, together with the ethical principles of beneficence and non-maleficence, is the most important rule in Medical Oaths at the present time. However, the scientific-technical advances in medicine have made this rule one of the most controversial ones because of its exceptions. In consequence, the aim of the present paper is to comparatively analyze the rule of confidentiality in Medical Oaths of different places, times, origins and in different versions of the Hippocratic Oath in order to determine what should be kept a secret and with what degree of commitment (absolute or "prima facie"). Of the thirty six analyzed Oaths, twenty-seven manifest this rule and nine do not. No relation was found between the manifestation of this rule and the place, time, origin and different versions of the Hippocratic Oath. Most pledges suggest not to reveal what has been seen or heard during the medical act, the same as in the Hippocratic Oath. Seven texts point out that confidentiality should be absolute and four give exceptions in connection with beneficence and justice principles and the moral duty of causing no damage to third parties. Two pledges specify protection of privacy. In conclusion, today confidentiality is considered to be a moral duty for the benefit of the patient and out of consideration for his autonomy; however, at the present time in medicine the duty of keeping absolute secrecy is being reconsidered.