Background <p>Correlations between self-reported and clinician-rated severity of depression are often only moderate, but studies in adults have shown that they increase over time. This study explored whether a similar effect occurs in adolescents and whether initial agreement vs. disagreement between self- and clinician ratings predicts differential outcomes.</p> Method <p>The analyzed data stem from a randomized controlled trial (DeLight) that explored the effect of bright light therapy as an add-on to treatment as usual in an inpatient sample of adolescents (<i>n</i> = 224). Depression was assessed at four time points (baseline, 4 weeks, 16 weeks, 28 weeks) using the Beck Depression Inventory (BDI-II) as a self-report measure and the Children’s Depression Rating Scale - Revised (CDRS-R) as a blinded clinician rating.</p> Results <p>The correlation between self- and clinician ratings was only moderate at baseline (<i>r</i> = .40) but increased considerably to a strong correlation at four weeks (<i>r</i> = .72), which was maintained thereafter. The predictive value of initial rater agreement / disagreement for the outcome was small (BDI-II) or non-significant (CDRS-R). Further analyses revealed that the reliability of both instruments increased over time and the factor structure became simpler, with fewer factors and higher factor loadings.</p> Conclusions <p>These findings indicate increasing homogeneity within self- and clinician ratings over time and suggest that some type of response shift occurred, with adolescents appearing to increasingly view their depression as a unified concept. A consideration of response shifts could lead to more accurate assessments of treatment effectiveness.</p>