The present study examines the clinical and research significance of the high frequency of multiple diagnoses emanating from the non-hierarchical descriptive approach to classification in the current psychiatric diagnostic systems. Data from a 15-year prospective cohort study of young adults from the general community were employed to evaluate the frequency of multiple disorders (i. e., multimorbidity), and the extent to which patterns of multiple disorders are associated with indicators of severity of psychopathology. The average number of lifetime disorders in this community-based sample was 2.1 with a range from 0 to 7. Associations within diagnostic spectra were more common than those between diagnostic spectra. The results confirm the link between comorbidity and severity demonstrated in several previous studies and further show that there is a direct increase in nearly all of the indicators of severity by the number of disorders for which the subjects met criteria across 15 years. Each of the major diagnostic categories, particularly depression, contributed to increased severity rather than representing a non-specific effect of the number of disorders. These findings demonstrate the importance of characterization of multiple syndromes rather than applying arbitrary hierarchical distinctions between diagnostic categories.