Many psychiatric patients seem to suffer from a multitude of psychiatric disorders. The term 'comorbidity' was introduced to characterise that situation. This term, however, is multi-interpretable and thus conceals more than it clarifies. Five interpretations of the concept of comorbidity are discussed, in conjunction with the consequences their validity would have for psychiatric research, in particular for the biological branch. If the term comorbidity continues to be used without further qualification, it will slow down the process of conceptualization of novel research strategies in experimental psychiatry, which is so urgently needed.