An international, multicenter, placebo-controlled study was undertaken to determine the safety and antidepressant efficacy of moclobemide, a new reversible inhibitor of monoamine oxidase A, and imipramine in the treatment of dysthymia (DSM-III-R). A total of 315 patients were enrolled and randomly assigned to an 8-week treatment in one of three groups (moclobemide, imipramine and placebo). Patients were male or female outpatients aged between 18 and 65 years meeting DSM-III-R criteria for dysthymia, primary type, with late or early onset. Of the patients in each group 85% completed the 8-week treatment period. The percentage of patients who no longer fulfilled DSM-III-R symptom criteria at treatment endpoint was significantly higher in the moclobemide (60%) and imipramine (49%) treatment groups than in the placebo group (22%). Differences to placebo were also statistically significant both for moclobemide and for imipramine on the other efficacy variables (i.e. Hamilton Rating Scale for Depression, final overall efficacy assessment, Clinical Global Impression and symptom check list self-rating). A significant superiority of moclobemide and imipramine over placebo was found in pure dysthymia and in double-depression, as well as in early and late onset subgroups. In early onset cases, moclobemide was significantly more effective than was imipramine on the Hamilton Rating Scale for Depression. Anticholinergic symptoms and sleepiness were significantly more frequent side effects on imipramine than on moclobemide or on placebo, and the investigators' final overall assessment of tolerability significantly favoured moclobemide over imipramine. This study demonstrates the efficacy of high dose moclobemide (mean dose 675 mg/day) and high dose imipramine (220 mg/day) against placebo in the treatment of dysthymia. Moclobemide was better tolerated than was imipramine.