BACKGROUND: The aim was to compare the efficacy and tolerability of mirtazapine with those of paroxetine. METHOD: 275 outpatients with a diagnosis of major depressive episode (DSM-IV) and a score > or = 18 on the 17-item Hamilton Rating Scale for Depression (HAM-D-17) were randomly assigned to 6 weeks of treatment with mirtazapine (15-45 mg/day) or paroxetine (20-40 mg/day). Efficacy was assessed by the HAM-D-17, Hamilton Rating Scale for Anxiety (HAM-A), and Clinical Global Impressions scales (Severity and Improvement), and analyses were performed on the intent-to-treat sample (127 mirtazapine-treated patients and 123 paroxetine-treated patients). RESULTS: Mean daily doses were 32.7 mg of mirtazapine and 22.9 mg of paroxetine. Thirty patients in the mirtazapine group and 33 in the paroxetine group dropped out. Both drugs were equally effective in reducing symptoms of depression. At week 1, the mean HAM-D-17 total score was significantly lower in mirtazapine- than paroxetine-treated patients (16.5 vs. 18.8, p = .0032). Similarly, significantly more mirtazapine-treated patients were HAM-D-17 responders (> or = 50% decrease from baseline) at weeks 1 (23.2% vs. 8.9%, p = .002) and 4 (58.3% vs. 44.5%, p = .04). Both treatments were equally effective in reducing anxiety. However, the reduction in mean HAM-A total score was significantly greater with mirtazapine than with paroxetine at week 1 (-5.1 vs. -3.5, p = .0435). Tolerability of both treatments was good, with more nausea, vomiting, tremor, and sweating in the paroxetine group and more weight increase and influenza-like symptoms in the mirtazapine group. CONCLUSION: Mirtazapine and paroxetine were equally effective after 6 weeks of therapy and were both well tolerated. A potentially faster onset of overall therapeutic efficacy of mirtazapine was suggested by significant differences between treatments after 1 week of therapy that were due to slightly larger improvements of several core symptoms of depression as well as distinct prevention of treatment-emergent worsening of anxiety and physical components of depression.