OBJECTIVE: The 200 units/mL formulation of insulin degludec (IDeg 200 units/mL) contains equal units of insulin in half the volume compared with the 100 units/mL formulation. We compared the efficacy and safety of IDeg 200 units/mL once daily with 100 units/mL insulin glargine (IGlar) in insulin-naive subjects with type 2 diabetes (T2DM) inadequately controlled with oral antidiabetic drugs. RESEARCH DESIGN AND METHODS: In this 26-week, open-label, treat-to-target trial, subjects (n = 457; mean HbA1c 8.3% [67 mmol/mol], BMI 32.4 kg/m(2), and fasting plasma glucose [FPG] 9.6 mmol/L [173.2 mg/dL]) were randomized to IDeg 200 units/mL or IGlar, both given once daily in combination with metformin with or without a dipeptidyl peptidase-4 inhibitor. Basal insulin was initiated at 10 units/day and titrated weekly to an FPG target of <5 mmol/L (<90 mg/dL) according to mean prebreakfast self-measured blood glucose values from the preceding 3 days. RESULTS: By 26 weeks, IDeg reduced HbA1c by 1.30% and was not inferior to IGlar. Mean observed FPG reductions were significantly greater with IDeg than IGlar (-3.7 vs. -3.4 mmol/L [-67 vs. -61 mg/dL]; estimated treatment difference: -0.42 [95% CI -0.78 to -0.06], P = 0.02). Despite this difference, rates of overall confirmed hypoglycemia were not higher with IDeg than with IGlar (1.22 and 1.42 episodes/patient-year, respectively), as were rates of nocturnal confirmed hypoglycemia (0.18 and 0.28 episodes/patient-year, respectively). Mean daily basal insulin dose was significantly lower by 11% with IDeg 200 units/mL compared with IGlar. IDeg was well-tolerated, and the rate of treatment-emergent adverse events was similar across groups. CONCLUSIONS: In this treat-to-target trial in insulin-naive patients with T2DM, IDeg 200 units/mL improved glycemic control similarly to IGlar with a low risk of hypoglycemia.