Video abstract: View a video abstract for this article. AIMS: This multicentre (N = 104), randomized controlled phase 4 study compared the efficacy and safety of insulin glargine 300 units/mL (Gla-300) with insulin glargine 100 units/mL (Gla-100) in patients with type 1 diabetes (T1D). MATERIALS AND METHODS: Patients were randomized 1:1 to self-perform morning Gla-300 or Gla-100 injections daily for 16 weeks. The primary endpoint was percentage of time blood glucose remained in the target range (70-180 mg/dL) during Week 15/16, measured by blinded continuous glucose monitoring. Secondary endpoints included incidence and rate of nocturnal symptomatic hypoglycaemia (=70 mg/dL), glycaemic variability parameters and safety assessments. Exploratory analyses were performed in patients with glycated haemoglobin (HbA1c) <7.5% at Week 16. RESULTS: Overall, 638 patients with T1D were included (Gla-300, n = 320; Gla-100, n = 318). In the modified intent-to-treat (mITT) population, no differences between Gla-300 and Gla-100 were observed in time in range, in glycaemic variability, or in incidence or rates of nocturnal symptomatic hypoglycaemia. In exploratory analyses of patients with HbA1c <7.5% at Week 16, Gla-300 recipients had greater improvement in time in range over 24 hours, during the day and at night compared with Gla-100 recipients (P < 0.05), with small increases in overall hypoglycaemia. CONCLUSIONS: Time in range and glycaemic variability were similar for Gla-300 and Gla-100 recipients at the end of study in the mITT population of relatively well-controlled patients with T1D. In patients with end-of-study HbA1c <7.5%, exploratory analyses suggested that Gla-300 provided improvements in time in range compared with Gla-100.