Background: The aim of the study was to examine the association between anxious/withdrawn behaviours at age 8 and the development of internalising disorders in adolescence and young adulthood (ages 16-21). Methods: Data were gathered over the course of a 21-year longitudinal study of a birth cohort of over 1,000 New-Zealand-born young people. Over the course of the study, data were gathered on: (a) anxious/withdrawn behaviour at age 8; (b) anxiety disorders and major depression at ages 16-18 and 18-21; (c) a range of potential confounding factors, including measures of adversity and family factors during childhood. Results: Increasing anxious/withdrawn behaviour at age 8 was associated with increasing risks of social phobia; specific phobia; panic/agoraphobia; and major depression during adolescence and young adulthood. These associations persisted after adjustment for a series of confounding social, childhood, and family factors. Conclusions: Anxious/withdrawn behaviour in childhood is associated with elevated rates of anxiety disorders and major depression during adolescence and young adulthood. These data are consistent with, and extend, previous clinical and epidemiologic findings, by showing consistent linkages between childhood anxious/withdrawn behaviours at age 8 and increased risk for anxiety disorders and depression at ages 16-18 and 18-21 among young persons in the community. These results may point to the need to provide support and possibly intervention to children showing early anxious or withdrawn behaviours.