This multicenter, open-label study with a duration of 85 days was performed to evaluate the antidepressant efficacy and safety of mirtazapine (dose range, 30-45 mg) in 12-18-year-old adolescents diagnosed with major depression. Twenty-four (24) patients (15 female patients and 9 male patients) meeting the DSM-IV criteria for major depression and the Hamilton Rating Scale for Depression (HAM-D-17) score of 18 at baseline were enrolled in the study. The primary outcome measures were HAM-D-17, Beck Depression Inventory (BDI), and Clinical Global Impression (CGI) scales. Any changes in symptoms of anxiety were measured using the Hamilton Anxiety Rating Scale (HAM-A). The average age of the 23 subjects, who were eligible for analysis, was 16.3 years (standard deviation (SD) 6.11, median 17.3). The mean daily dose of mirtazapine was 32.9 mg. Mirtazapine showed a marked efficacy on all rating scales and was well tolerated. Mirtazapine had a beneficial effect on sleep. A rapid onset of sleep and pattern of action was seen. No dropouts due to adverse events were recorded. The most common treatment-emergent adverse events were tiredness, increased appetite, and dizziness. The results of this study suggest that mirtazapine may be an effective treatment for major depression in adolescents.