Long-term safety and the effects of a St. John's wort (SJW) extract Ze 117 (Hypericum perforatum) were evaluated in the treatment of patients with depression. An open multicentre safety study with 440 out-patients suffering from mild to moderate depression according to ICD-10 was conducted. Patients were treated for up to 1 year with 500 mg St. John's wort extract per day (Ze 117). Evaluation criteria were safety (adverse event frequency) and influence on depression (HAM-D, CGI). Two hundred and seventeen (49%) patients reported 504 adverse events, 30 (6%) of which were possibly or probably related to the treatment. Gastrointestinal and skin complaints were the most common events associated with treatment. No age-related difference in the safety of the applied medication was found. The long-term intake of up to 1 year of the study medication did not result in any changes in clinical chemistry and electrocardiogram recordings. Body mass index (BMI) did not change either. Mean HAM-D scores decreased steadily from 20.58 at baseline to 12.07 at week 26 and to 11.18 at week 52. Mean CGI scores decreased from 3.99 to 2.20 at week 26 and 2.19 at week 52. Therefore, St. John's wort extract ZE 117 is a safe and effective way to treat mild to moderate depression over long periods of time, and therefore seems especially suitable for a relapse prevention.