OBJECTIVE Depression and anxiety are highly prevalent and frequently comorbid mental health disorders that often manifest during childhood and adolescence. The purpose of this study was to prospectively examine potential common underlying mechanisms predicting the development of depressive and anxiety symptoms to identify mutual underlying modifiable risk factors to target with mental health promotion and prevention interventions. METHODS A community-based sample of 11- to 15-year-old adolescents participated in a prospective, longitudinal pilot study with three waves of data collection over a 6-month period. RESULTS At baseline, stressful life events, worry, and rumination accounted for 35% of the variance in anxiety scores (adjusted R(2) = 0.28) and 58% of the variance in depressive symptoms (adjusted R(2) = 0.53), while at Time 3 (T3; 6 months post baseline) these variables accounted for 63% of the variance in anxiety scores (adjusted R(2) = 0.59) and 45% of the variance in depressive symptoms (adjusted R(2) = 0.39). After controlling for baseline values, increases in stress, worry, and rumination were predictive of both depressive and anxiety symptoms at T3. CONCLUSIONS Study findings point to the importance of prevention and early intervention programs including content addressing stress management, rumination, and worry.