The objective of this study was to assess the efficacy and safety of standardized extract SHR-5 of rhizomes of Rhodiola rosea L. in patients suffering from a current episode of mild/moderate depression. The phase III clinical trial was carried out as a randomized double-blind placebo controlled study with parallel groups over 6 weeks. Participants, males and females aged 18-70 years, were selected according to DSM-IV diagnostic criteria for depression, the severity of which was determined by scores gained in Beck Depression Inventory and Hamilton Rating Scale for Depression (HAMD) questionnaires. Patients with initial HAMD scores between 21 and 31 were randomized into three groups, one of which (group A: 31 patients) received two tablets daily of SHR-5 (340 mg/day), a second (group B: 29 patients) received two tablets twice per day of SHR-5 (680 mg/day), and a third (group C: 29 patients) received two placebo tablets daily. The efficacy of SHR-5 extract with respect to depressive complaints was assessed on days 0 and 42 of the study period from total and specific subgroup HAMD scores. For individuals in groups A and B, overall depression, together with insomnia, emotional instability and somatization, but not self-esteem, improved significantly following medication, whilst the placebo group did not show such improvements. No serious side-effects were reported in any of the groups A-C. It is concluded that the standardized extract SHR-5 shows anti-depressive potency in patients with mild to moderate depression when administered in dosages of either 340 or 680 mg/day over a 6-week period.