Of 580 patients randomly assigned to short-term, double-blind treatment with either mirtazapine, amitriptyline or placebo, a total of 217 patients clinically judged to be responders subsequently continued on the same medication for up to 2 years in the long-term treatment study (mirtazapine, n = 74; amitriptyline, n = 86 and placebo, n = 57). The efficacy of mirtazapine in relapse prevention was seen in an analysis of the first 20 weeks data. Significantly fewer patients relapsed during treatment with mirtazapine compared with placebo (p < 0.05), and a significantly longer time to relapse was shown on the survival analysis. There was a significant advantage for amitriptyline compared with placebo in the first 20 weeks, with fewer patients relapsing. There was a significant advantage for mirtazapine compared with amitriptyline at 20 weeks seen on the survival analysis (p < 0.05). The significant advantage for mirtazapine compared with placebo was also seen in the prophylactic phase of treatment after 20 weeks. At the endpoint there were significantly more patients in the placebo group with a return of symptoms and significantly fewer showing sustained response. Amitriptyline was better than placebo with fewer patients suffering a recurrence of symptoms, but there was no difference from placebo in the proportion of patients with sustained response. Mirtazapine was well tolerated with a side-effect profile similar to that of placebo. The only adverse event reported significantly more frequently on mirtazapine than on placebo was weight gain. Objectively measured weight gain was more frequent with amitriptyline (22% of patients) compared with mirtazapine (13% of patients). Amitriptyline was associated with significantly more adverse events than either mirtazapine or placebo, in particular sedative and anticholinergic side effects. The efficacy of mirtazapine in reducing the risk of relapse and the recurrence of depression, which on some measures showed an advantage compared with amitriptyline, coupled with its improved side-effect profile, commends this antidepressant for the long-term treatment of depression.