Agomelatine is efficacious in reducing symptoms and preventing relapse in placebo-controlled trials in generalised anxiety disorder (GAD). Nevertheless, fixed dose studies of agomelatine in GAD have not been undertaken. To determine the minimally effective optimal dose of agomelatine in GAD, the efficacy of two doses of agomelatine (10 and 25 mg/day) was investigated in a 12-week, placebo-controlled, double-blind, international study in patients with a primary diagnosis of GAD. The primary outcome measure was the Hamilton Anxiety scale (HAM-A). The study was undertaken in 35 clinical centers in Finland, Russia, Poland, Slovakia and Ukraine from August 2013 to January 2015. 131 out-patients were included in the agomelatine 10 mg group, 139 in the agomelatine 25 mg group, and 142 in the placebo group. Both doses of agomelatine were associated with significant decreases in the HAM-A at week 12 (difference versus placebo of 7.16±1.00 at 10 mg and 11.08±0.98 at 25 mg, p<0.0001). Significant effects on all secondary measures were found for both doses at week 12; including psychic and somatic HAM-A subscales, response rate, remission on the HAM-A, and functional impairment. Findings were confirmed in subsets of more severely ill patients on all endpoints. The low placebo response rate observed in this study was consistent with an increase in the quality of data collected. Agomelatine was well-tolerated by patients, with minimal distinctions from placebo. There was a dose effect of agomelatine, with a greater placebo-agomelatine difference in the agomelatine 25 mg group, compared to the agomelatine 10 mg group.The present data support early work indicating the efficacy and tolerability of agomelatine in the treatment of GAD.