To evaluate the effectiveness of interpersonal therapy (IPT) implemented by well-supervised, novice IPT therapists in treating adolescents with moderate to severe mood disorders of lengthy duration. Twenty-five adolescents with moderate to severe major depression, lasting an average of 8 months, received 12 weeks of IPT. All participants were assessed with the Schedule for Affective Disorders and Schizophrenia for School-Age Children-Present and Lifetime Version, the Beck Depression Inventory (BDI), the Hamilton Rating Scale for Depression (HRSD), and the Children's Global Assessment Scale (C-GAS) at baseline and follow-up. The majority of participants improved substantially on the BDI, the HRSD, and the C-GAS; 84% met remission criteria on the HRSD (score < 7); and 80% met remission criteria on the BDI (score < 10). Results demonstrate that IPT is effective in treating moderately to severely depressed adolescents, that IPT is effective with depression lasting several months, and that IPT can be effectively implemented by well-supervised clinicians with no prior training in IPT.