Objectives: While hospitalization is an essential aspect of the therapeutic strategy for adolescents with severe or treatment-refractory mood disorders, little is known about the outcome predictors during inpatient treatment. Methods: A retrospective chart review was conducted in a university tertiary referral hospital to determine the factors associated with the length of stay, symptom improvement (based on the Clinical Global Impression-Improvement [CGI-I] scale), and the change in the overall level of functioning during the stay (based on the Children-Global Assessment Scale [CGAS]). Over 2 years, 106 adolescents were diagnosed with mood disorders (mean age = 15.0 +/- 0.16; 43% girls), with a particular high rate of associated adverse psychosocial factors, and an average length of stay longer than most psychiatric hospital settings (mean = 100.7 +/- 9.57 days). Results: Multivariate analysis concluded that longer duration of current episode and worse functioning at admission (CGAS score) were independent predictors for length of stay. Greater functional improvement (CGAS score change from admission to discharge) was best predicted by the diagnosis of bipolar disorder, lower functioning, and greater illness severity on admission. Symptom improvement (CGI-I) did not have any independent predictors. Conclusion: This finding supports the value of measuring symptoms duration in predicting the hospitalization outcomes of adolescents with severe or treatment-refractory mood disorders, in view of addressing maintenance factors at an early stage. Bipolar symptoms should be sought not only at admission but also regularly during the stay.