Description of psychopathological symptoms in mentally retarded subjects is of great heuristic value because of the increasingly observed regular association with a known genetic disorder as well as the heterogenous and atypical phenomenology. In psychiatric diagnostic procedures, it is assumed that symptoms can be put together in diagnostic entities with a presumed specific etiopathogenesis. In mentally retarded subjects, on the contrary, various syndromes with a specific genetic etiology present with a confusing array of psychiatric manifestations. The concept of behavioural phenotypes postulates a specific relationship between gene defect and behaviour, although considerable overlap between syndromes and differences within syndromes concerning behavioural manifestations are frequently observed. Since syndrome specific psychiatric disorders can be established too, the term psychopathological phenotype seems to be appropriate. The making of a psychiatric diagnosis is therefore a complex task which requires expertise on several domains.