Background: Growing evidence indicates that anxiety and depressive disorder onset may involve a prodromal buildup of symptoms. Also, stressful life events may precipitate gradual symptom increase, leading to the development of full-blown disorders. This study used prospective longitudinal data to examine the theory that, over time, stressful events, such as parent- adolescent disagreements, influence the longitudinal course of adolescents' internalizing symptoms, which in turn predict first onset of a depressive or anxiety disorder. Methods= A community sample of 303 families with an adolescent aged 12 or 13 years in year 1 provided repeated measures of parent-adolescent disagreements and adolescents' internalizing symptoms over 3 and 4 years, respectively. At age 19 or 20 years, the adolescents were assessed for anxiety and depressive disorders using structured interviews based on DSM-III-R criteria. The hypothesized associations were estimated using latent growth curve modeling procedures. Results: Year 1 parent- adolescent disagreements predicted year 1 adolescents' internalizing symptoms, and changes in disagreements from year 1 to year 3 predicted changes in internalizing symptoms from year 1 to year 4. Both the year 1 level and changes in symptoms predicted internalizing disorder onset in years 4 through 7, and both the year 1 level and changes in disagreements indirectly predicted disorder onset. Conclusions: Among adolescents, persistent or escalating stressful events, such as disagreements with parents, indirectly increase the risk for internalizing disorder onset through their direct association with high or increasing symptom levels. Chronically high or increasing symptom levels directly increase risk for internalizing disorder.