AimsTo evaluate the efficacy and safety of insulin glargine 300U/mL (Gla-300) versus glargine 100U/mL (Gla-100) in adults with type 1 diabetes in Japan over 12months. MethodsEDITION JP 1 was a multicentre, randomised, open-label phase 3 study. Following a 6-month on-treatment period, participants continued to receive Gla-300 or Gla-100 once daily, plus mealtime insulin, over a 6-month open-label extension phase. HbA1c, hypoglycaemia, body weight and adverse events were assessed. ResultsOverall, 114/122 (93%) and 114/121 (94%) of participants in the Gla-300 and Gla-100 group, respectively, completed the 6-month extension phase. Glycaemic control was sustained in both groups up to month 12 (mean HbA1c: Gla-300, 7.9% [62mmol/mol]; Gla-100, 7.8% [62mmol/mol]). Annualised rates of hypoglycaemia were lower with Gla-300 versus Gla-100; significantly for nocturnal confirmed (<3.0mmol/L [<54mg/dL]) or severe hypoglycaemia (2.39 and 3.85 events per participant-year; rate ratio: 0.62 [0.39–0.97]). No between-treatment differences in mean body weight change or adverse events were observed. ConclusionOver 12months’ treatment, participants with type 1 diabetes receiving Gla-300 achieved sustained glycaemic control and experienced less nocturnal hypoglycaemia that was confirmed (<3.0mmol/L [<54mg/dL]) or severe compared with Gla-100, supporting the 6-month results.