The aim of this study was to evaluate the dose-dependent effect of dulaglutide, a glucagon-like peptide-1 receptor agonist, on glycaemic control in Japanese patients with type 2 diabetes mellitus who were treated with diet/exercise or oral antidiabetic drug monotherapy. In this randomised, double-blind, placebo-controlled, parallel-group, 12-week study, patients received once weekly subcutaneous dulaglutide doses of 0.25, 0.5, or 0.75 mg (DU 0.25, DU 0.5, and DU 0.75, respectively) or placebo (n=36, 37, 35, and 37, respectively). The primary measure was change from baseline in glycated haemoglobin (HbA1c; %) at 12 weeks. Continuous variables were analysed using a mixed-effects model for repeated measures. Significant dose-dependent reductions in HbA1c were observed (least squares mean difference versus placebo [95% confidence interval]): DU 0.25=-0.72% (-0.95, -0.48), DU 0.5=-0.97% (-1.20, -0.73), and DU 0.75=-1.17% (-1.41, -0.93); p<0.001. Significant improvements in plasma glucose (PG), both fasting and average 7-point self-monitored blood glucose, were also observed with dulaglutide versus placebo (p<0.001). Dulaglutide was well-tolerated. Gastrointestinal adverse events (AEs) were more common in dulaglutide-treated patients, with nausea the most frequent (8 [5.5%]). Few dulaglutide-treated patients discontinued due to AEs (4 [3.7%]), and no serious AEs related to study medication occurred. Three patients (DU 0.5=1 and DU 0.75=2) reported asymptomatic hypoglycaemia (PG ≤70 mg/dL). The observed dose-dependent reduction in HbA1c and acceptable safety profile support further clinical development of dulaglutide for treatment of type 2 diabetes mellitus in Japan.