Prior research findings had suggested that adolescents with anxiety disorders might be at risk of ongoing internalizing mental health problems. The present study reported the results of a 21-year longitudinal study of the relationship between adolescent anxiety disorder and later mental health, educational, and social functioning in a birth cohort of 964 New Zealand children. The specific aims of the study were: (1) to document associations between the extent of anxiety disorder in adolescence (14- to 16-years-old) and a range of later mental health, educational, and social functioning outcomes measured in late adolescence and early adulthood (16- to 21-years-old); and (2) to examine whether associations between the extent of anxiety disorder and later outcomes persisted after statistical adjustment for a range of perplexing social background, family, and individual factors. The results extended the empirical understanding of the longer-term effects of adolescent anxiety disorders. The present findings suggested that anxiety-disordered adolescents had elevated rates of anxiety and depression as young adults. In addition, there was evidence suggesting that adolescent anxiety was associated with a number of other adverse mental health and life course outcomes. As the number of anxiety disorders increased, there was also an increase in the rates of later nicotine, alcohol, and illicit drug dependence; suicidal behavior; educational underachievement; and early parenthood. However, it was also found that adolescents with anxiety disorders were characterized by a number of socio-familial and personal disadvantages that might account for the difficulties faced as young adults. The elevated rates of these outcomes were suggested as a consequence of the risk factors and life processes associated with anxiety rather than the direct effects of early-onset anxiety on later life course development. Future research in this area suggested the development of a better understanding of the underlying process or processes by which anxiety disorders increase an individual’s risk of developing other mental health problems such as depression and illicit drug dependence, and which also deter them from seeking higher educational qualifications. Tables and references