Increasing numbers of people are found to have two or more diseases at the same time, which is termed multimorbidity. We studied the prevalence, incidence, and determinants of multimorbidity and the statistical clustering of chronic and recurrent diseases in a general practice setting. Prevalence of multimorbidity increased with all age groups from 10% in the 0-19-year-old age group up to 78% in subjects aged 80 and over. Increasing age, lower level of education, and public health insurance were related to the occurrence of morbidity, but even more strongly to the occurrence and degree of multimorbidity. The one-year incidence of multimorbidity (the new occurrence of two or more diseases in one year) was related to increasing age, public health insurance, and the presence of prevalent diseases at baseline. Statistical clustering of diseases was stronger than expected, especially among the younger subjects.