[Comorbidity in psychiatric diseases. Theoretical considerations].

During the last 10 years, comorbidity has become an important topic in psychiatric research, leading to a growing number of publications. We discuss the theoretical and historical background of this development and present definitions and models for the phenomenon. Methodological difficulties have to be stated: considering its clinical consequences, we conclude that (1) comorbidity is an inevitable methodological consequence of the diagnostic strategies and continued diversification of DSM-IV and ICD-10, (2) there is little doubt of its relevance in the clinical context, and (3) the diagnostic strategies of DSM-IV and ICD-10 concerning comorbidity are difficult to follow in clinical reality.