Background. Although the diagnosis and treatment of depression among adolescents are among the top public health priorities, there is little epidemiological data available for this population. Objectives. The objectives of this study were to estimate the prevalence of depressive disorders among ninth-grade middle school students and to identify the main sociodemographic variables associated with these disorders. Method. Data were collected from ninth-grade students during the three-year cycle of school-based surveys in 2003–2004. Depressive disorders were assessed according to the DSM-IV classification using a self-report questionnaire derived from a standardized tool, the Composite International Diagnosis Interview-Short Form (CIDI-SF). Results. The national sample included 7,110 students with a mean age of 15.1 years. Among the adolescents surveyed, 9.6% (95% CI: 8.6–10.6) reported symptoms consistent with at least one major depressive episode in the past 12 months. Three factors were independently associated with a higher probability of experiencing a major depressive episode in the past 12 months: being female (adjusted odds ratio = 3.0; 95% CI: 2.4–3.8), being 16 years of age or older (aOR = 2.2; 95% CI: 1.0–5.0), and living with one’s mother alone (aOR = 1.7; 95% CI: 1.3–2.4) or with one’s mother and her spouse (adjusted OR = 1.4; 95% CI: 1.0–2.1). Conclusion. Following a discussion of these results, obtained from a representative national sample demonstrating the prevalence of depressive disorders among adolescents, there is a clear need to establish mental health surveillance in this population, as well as appropriate prevention and treatment strategies.