It is the nature of a disease that it may not only improve but also deteriorate even when under medical treatment. Medical treatment thus constitutes a high-risk activity. The great advances in medical diagnosis and treatment in recent years have increased this risk still further. The medical profession is becoming increasingly unsettled and worried about claims of treatment errors, not least because-of the increasing pressure from the public and media. This has wrongly led to the development of "defensive medicine" such as has already started to become established in the USA. The medical expert should stand as a mediator between medicine and law, and thus be in a position to clear up misunderstandings between the two disciplines with their different mind sets. For these reasons, a forensic specialist should be appointed as an expert whenever possible; he may be proposed to the examining judge as an additional expert in the field in question to answer special questions put to him. This article discussed the basis on which an expert medical opinion should be provided in relation to an injury arising as a result of a treatment.