OBJECTIVES:  This systematic review evaluated the real-world clinical effectiveness of switching to insulin degludec (IDeg) in Japanese patients. METHODS: Studies were identified from Japanese Diabetes Society (JDS) abstracts (2014-2015) and PubMed (2012 onwards). Inclusion criteria were: Japanese population, >15 participants and only studies switching patients from basal or basal–bolus insulin regimens to IDeg. Randomised controlled trials and case reports were excluded. Weighted mean changes in safety and effectiveness endpoints were calculated using the number of patients in each study. Statistical significance: p≤0.05. RESULTS: A total of 81 JDS abstracts and 5 manuscripts met the search criteria, representing 4238 patients (1028 T1D, 602 T2D, 2608 not specified). HbA was reported in 93% of studies and was improved in 84% (51% significant, 33% numerical), unchanged in 12% and worse in 4% (3% numerical, 1% significant). Across all studies the weighted mean actual change in HbA was –0.25%. Basal insulin dose was reported in 57.5% of studies and was lower in 60% (30% significant, 30% numerical), numerically unchanged in 26% and higher in 14% (2% significant, 12% numerical). The weighted mean change in basal insulin dose was –4.81% and –3.72% for all studies and for significant-only studies, respectively. The weighted mean change in basal dose based on all studies was –8,9%, –5.5% and –2.9% for T1D, T2D and unspecified populations, respectively. Hypoglycaemia was recorded in 28.1% of the studies. After switching, 54.8% of studies reported decreased hypoglycaemia, 29.0% no change and 16.1% an increase. Quality of life (QoL) was measured in 11.1% of studies. 81.8% reported improved QoL after switching, 18.2% reported no change in QoL. CONCLUSIONS: Switching from a conventional basal insulin to IDeg has the potential to improve HbA with a lower insulin dose. Switching to IDeg may also provide a reduced risk of hypoglycaemia and improvement in QoL.