Aim: Comorbidity is increasingly prevalent. Moreover, many different definitions and interpretations of this phenomenon are used. Because of its social and clinical significance, it is important that the concept is clear. In this review a summary of current uses of this concept and relevant figures are given, as well as a proposal for future use. Method: A computerised search and an additional literature search were performed to identify relevant papers on comorbidity and related subjects. Available literature on the subject (nequals;38) was reviewed on definition, operationalisation, occurrence and potential determinants. Results: The definitions in use are ambiguous; while some just imply coexistence of several diseases, others require medical conditions additional to an index disease. Other differences are related to the population and the number and type of diseases under study. Reported prevalence of comorbidity varies from less than 1% to over 50%, with higher prevalence among the elderly. Consequences such as disability and decreased quality of life are reported for the individual. There are also effects on the family and close friends which can be expressed in the social prevalence of disease. In addition there are consequences for the economy and health care. Explanations for observed comorbidity include causal mechanisms, bias and a general susceptibility for disease. Discussion: Since the introduction of the term comorbidity about 30 years ago, the concept has been used in many different ways. Because of the ambiguity of the term, we suggest a distinction between comorbidity based on the ‘classical’ definition (implying an index disease) and multimorbidity, meaning any co-occurrence of medical conditions within a person. Eur J Gen Pract 1996; 2: 65–70.

Time to focus on the needs of this vulnerable and growing population