The effect of readiness to change on treatment outcome was examined among 332 adolescents (46% male, 74% Caucasian), ages 12 through 17 years (M = 14.6, SD = 1.5), with major depressive disorder who were participating in the Treatment for Adolescents With Depression Study (TADS). TADS is a randomized clinical trial comparing the effectiveness of fluoxetine (an antidepressant medication), cognitive-behavioral therapy, their combination, and a pill placebo. An abbreviated Stages of Change Questionnaire was used to obtain 4 readiness to change scores: precontemplation, contemplation, action, and maintenance. The association between each readiness score and depression severity across 12 weeks of acute treatment for depression, as measured by the Children's Depression Rating Scale--Revised, was examined. Although treatment response was not moderated by any of the readiness scores, baseline action scores predicted outcome: Higher action scores were associated with better outcome regardless of treatment modality. Furthermore, treatment effects were mediated by change in action scores during the first 6 weeks of treatment, with increases in action scores related to greater improvement in depression. Assessing readiness to change may have implications for tailoring treatments for depressed adolescents. (Contains 5 tables and 1 figure.)