OBJECTIVE: The purpose of this study was to find the clinical characteristics of patients who had prematurely terminated mirtazapine treatment for their major depressive disorders in natural clinical settings, and the effect of sedation on the premature termination of treatment. METHODS: Patients who had been diagnosed with major depressive disorder by DSM-IV and administered mirtazapine at least one time were included in this study. Charts reviews were done for patients who had visited out patients clinic in Sungkyunkwan University Kangbuk Samsung hospital since March 1st 2000 to December 31st 2001. Baseline depression,, anxiety, initial, middle, and terminal insomnia, and suicidality were assessed with 17-item Hamilton Rating Scale for Depression (HAMD). After 8 weeks of treatment, patients were divided into maintained group and terminated group, and also patients group with sedation and without sedation. Age, initial dosage, duration of treatment, dosage of combined drug, baseline depression, anxiety, sleep characteristics, and suicidality were compared between these two groups. RESULTS: Terminated group had higher score compared to maintained group in middle insomnia before medication (t= -3.225, p<0.01). Patients with sedation were younger (t=2.321, p<0.05), and had shorter period of treatment than patients without sedation (t=2.717, p<0.01) significantly. CONCLUSION: In natural clinical setting, depressive patients who terminated mirtazapine treatment had had more difficulty in maintaining sleep. Sedation was reported by younger patients, and related with premature terminiation of mirtazapine treatment.