With the increasing number of elderly people in The Netherlands the prevalence of chronic diseases will rise in the next decades. It is recognized in general practice that many older patients suffer from more than one chronic disease (comorbidity). The aim of this study is to describe the extent of comorbidity for the following diseases: hypertension, chronic ischemic heart disease, diabetes mellitus, chronic nonspecific lung disease, osteoarthritis. In a general practice population of 23,534 persons, 1989 patients have been identified with one or more chronic diseases. Only diseases in agreement with diagnostic criteria were included. In persons of 65 and older 23% suffer from one or more of the chronic diseases under study. Within this group 15% suffer from more than one of the chronic diseases. Osteoarthritis and diabetes mellitus are the diseases with the highest rate of comorbidity. Comorbidity restricts the external validity of results from single-disease intervention studies and complicates the organization of care.