The mental disorder concept has not been paid due attention to. The aim of this paper is twofold: first, to assess how much space has been given to the mental disorder concept in textbooks of psychiatry, and second, to show in how many domains both within and beyond psychiatry the mental disorder concept plays a key role. A number of textbooks written in English, German, French, Spanish, and Italian, selected as examples, have been scanned so as to see if there is a chapter dealing with mental disorder, in particular with its definition. Also, the fields in which the mental disorder plays a major role have been identified, and the reasons why the concept of mental disorder is relevant for them have been explored. There is no chapter dealing with the definition of mental disorder in some textbooks of psychiatry in English, German, French, Spanish, and Italian that have been selected as examples. Yet there are numerous domains, directly or indirectly related to psychiatry, in which the mental disorder concept is a substantial element. The results show that the concept of mental disorder should be kept high on psychiatric agenda and given due space in textbooks of psychiatry accordingly.