Mixed anxiety and depressive disorder (MADD; ICD-10 F41.2) is a condition characterized by subsyndromal symptoms of anxiety and depression, neither of which are clearly predominant. Silexan has been demonstrated to be efficacious in subsyndromal and syndromal anxiety disorders and co-morbid depressive symptoms. In this study 318 adult out-patients with MADD according to ICD-10 criteria, a total score ≥18 points on the Hamilton Anxiety Rating Scale (HAMA), and at least moderately severe anxious and depressed mood were randomized and received 1×80mg Silexan or placebo in double-blind fashion for a scheduled period of 70 days. Primary outcome measures were the HAMA and Montgomery Åsberg Depression Rating Scale (MADRS) total score changes between baseline and treatment end. The HAMA total score decreased by 10.8±9.6 points for Silexan and by 8.4±8.9 points for placebo (treatment group difference: p<0.01, one-sided; ANCOVA with factors for treatment and centre and the baseline value as covariate), and total score decreases of 9.2±9.9 and 6.1±7.6 points, respectively, were observed for the MADRS (p<0.001). Compared to placebo, the patients treated with Silexan had a better over-all clinical outcome and showed more pronounced improvements of impaired daily living skills and health related quality of life. Eructation was the only adverse event with a substantially higher incidence under Silexan. The study thus demonstrates that Silexan is efficacious and safe in the treatment of MADD.