We have investigated the antidepressant efficacy and safety of Hypericum perforatum (St. John's wort) extract WS5572 in a double-blind, placebo-controlled multicenter clinical trial. 72 patients (WS 5572: 37, placebo: 35) with a diagnosis of mild to moderate major depressive disorder (according to DSM-IV criteria) were randomized in 42 days of treatment with either 300 mg WS5572 t.i.d. or placebo. The primary efficacy variable was the change of the 17-item Hamilton Depression Scale (HAMD) total score between baseline and double-blind treatment. The study was conducted with an adaptive interim analysis, which led to early stopping because convincing treatment efficacy could already be demonstrated. Group differences in favor of WS 5572 were descriptively apparent as early as day 7 of randomized treatment and were statistically significant at days 28 (p = 0.011) and day 42 (p < 0.001). Between baseline and treatment end, the HAMD total score decreased from 19.7 +/- 3.4 to 8.9 +/- 4.3 points in the Hypericum group and from 20.1 +/- 2.6 to 14.4 +/- 6.8 points in the placebo group (mean +/- SD). Responder rates were consistently higher in the Hypericum group. Comparable group differences in favor of WS 5572 were also found for von Zerssen's Depression Scale (D-S; self-rating), Clinical Global Impressions (CGI) and a global patient's self-assessment (GPA). Tolerability was very good in both groups, with no adverse drug reactions and no clinically relevant changes in safety parameters. The results indicate that Hypericum extract WS 5572 is an effective and well-tolerated drug for the treatment of mild to moderate major depressive disorder.