Background: This study evaluated the clinical efficacy of mirtazapine and its effect on serum brain-derived neurotrophic factor (BDNF) and tumor necrosis factor-α (TNF-α) levels in patients of major-depressive disorder (MDD) with severe depression. Methods: Patients (aged 18-60) with MDD diagnosed by DSM-IV criteria, and Hamilton Rating Scale for Depression (HAM-D) score ≥25 were included (n = 30). Mirtazapine was given in the doses of 30 mg/day. All patients were followed up for 12 weeks for the evaluation of clinical efficacy, safety along with serum BDNF and TNF-α levels. Results: HAM-D score at the start of treatment was 30.1 ± 1.92, which significantly (p < 0.05) reduced to 13.47 ± 1.77 at 12 weeks of treatment. In responders, mean serum BDNF levels at the start of treatment were 2.32 ± 0.3 ng/ml, which significantly (p < 0.05) increased to 2.79 ± 0.33 ng/ml at 12 weeks of treatment and mean serum TNF-α levels at the start were 5.18 ± 0.67 pg/ml, which significantly decreased to 4.36 ± 0.72 pg/ml (p < 0.05) at 12 weeks of treatment. Conclusion: Our results suggest that mirtazapine is effective and well tolerated in severely depressed patients and treatment response is associated with an increase in serum BDNF and a decrease in serum TNF-α levels.