The dearth of long-term follow-up studies of community-based samples and differences in methodology in existing studies highlight the need for research designed to examine the stability, comorbidity, and diagnostic thresholds of depression and anxiety in the community. Prospective study of a community-based cohort aged 19 and 20 years from the canton of Zurich, Switzerland. Semistructured diagnostic interviews were administered by clinically experienced interviewers at 5 assessment points during a 15-year period. The format of the interview permitted assessment of major mental disorders at both the diagnostic and subthreshold levels. Comorbid anxiety and depression tended to be far more persistent than either syndrome alone. Individuals with anxiety states alone tended to develop either depression alone or comorbid anxiety and depression as they progressed through adulthood. In contrast, depression alone and depression comorbid with anxiety tended to be more stable than anxiety alone over time. The patterns of stability were similar for subthreshold- and threshold-level disorders. These findings have important implications for classification and treatment of affective disorders. The greater stability of comorbid anxiety and depression than either disorder alone illustrates the importance of further investigation of comorbid states compared with noncomorbid states in etiologic and treatment research. The persistence of subthreshold-level depression and anxiety from early to mid adulthood also suggests the importance of characterizing the continuum of expression of depression and anxiety rather than adhering to strict diagnostic thresholds.