The paper presents the responsibilities of paediatricians with regard to the diagnosis and treatment they should provide in compliance with the principle of due diligence. This principle – fundamental for the medical profession – stems not only from the regulations of law but also from deontological standards. For this reason, in addition to criminal and civil liability, physicians also bear professional liability, which is addressed in this paper. In the Polish legal doctrine, the professional liability of physicians is often considered to be quasi-criminal, adapted to the needs of specific institutions. Consequently, it is included in the framework of criminal law in the broad sense. Nevertheless, proceedings concerning the professional liability of physicians are held independently of criminal or other disciplinary proceedings pertaining to the same act. This paper addresses aspects related to the operation of medical courts. The issue is particularly important in view of the fact that judges sitting in medical courts of the first and second instance independently decide on factual and legal issues. With respect to the adjudication process, members of medical courts are subject to the provisions of the generally applicable law. A key element in court proceedings is the testimony of expert medical witnesses. Medical experts called by the court as expert medical witnesses show the medical context of a legal dispute based on their theoretical knowledge and practical expertise. The paper highlights the form and the elements that should be included in the expert opinion. Crucially, the preparation of expert opinions requires access to appropriate materials related to the case. Expert medical witnesses serving as court-appointed experts in disciplinary cases acquire such materials based on their personal examination of the injured party and comprehensive review of available documentation – or solely on the basis of medical records. The paper presents examples of such expert opinions.