S-adenosyl-L-methionine (SAMe) is a natural substance which constitutes the most important methyl donor in transmethylation reactions in the central nervous system. Several clinical trials have shown that SAMe possesses an antidepressant activity. This multicentre study was carried out to confirm both efficacy and safety of SAMe in the treatment of major depression. SAMe was given intramuscularly (i.m.) at a dose of 400 mg/d, double-blind, vs. 150 mg/d oral Imipramine (IMI) in patients with a diagnosis of major depressive episode, with a baseline score on the 21-item Hamilton Depression Rating Scale (HAMD) of >or=18. A total of 146 patients received SAMe whereas 147 received IMI for a period of 4 wk. The two main efficacy measures were endpoint HAMD score and percentage of responders to Clinical Global Impression (CGI) at week 4. Secondary efficacy measures were the final Montgomery-Asberg Depression Rating Scale (MADRS) scores and the response rate intended as a fall in HAMD scores of at least 50% with respect to baseline. The analysis of safety and tolerability was conducted in all treated patients. SAMe and IMI did not differ significantly on any efficacy measure, either main or secondary. Adverse events were significantly less in patients treated with SAMe compared to those treated with IMI. These data show 400 mg/d i.m. SAMe to be comparable to 150 mg/d oral IMI in terms of antidepressive efficacy, but significantly better tolerated. These findings suggest interesting perspectives for the use of SAMe in depression.