AIMS/INTRODUCTION: This study aimed to investigate the risk of diabetic ketoacidosis (DKA) in insulin-treated type 1 diabetes (T1D) patients administered Sodium-glucose cotransporter 2 (SGLT2) inhibitors in real-world clinical practice. MATERIALS AND METHODS: We conducted a real-world, retrospective, observational cohort study using Japanese Medical Data Vision, a diagnosis procedure combination database. We identified insulin-treated adult T1D patients enrolled from December 2018 to October 2019. We assessed the incidence and risk of DKA in T1D patients using SGLT2 inhibitors in an "on-label" manner. Cox multivariate regression analyses were performed to determine clinical factors linked to SGLT2 inhibitor-associated DKA. RESULTS: Of 11,475 T1D patients, 1,898 (16.5%) were prescribed SGLT2 inhibitors. DKA occurred in 139 (7.3%) of these patients, with 20.2 incidences per 100 person-years. These patients also showed significantly higher DKA rates than did those not receiving SGLT2 inhibitors (hazard ratio, 1.66; 95% CI, 1.33-2.06; p <0.001). The mean time until DKA onset in SGLT2 inhibitor-treated T1D patients was 30.6 (±30.1) days. The risk of SGLT2 inhibitor-associated DKA increased in T1D patients irrespective of sex, age, or body mass index (BMI). However, the risk did not increase in T1D patients receiving continuous subcutaneous insulin infusion (CSII), which warrants further investigation because of the small number of T1D patients receiving CSII. CONCLUSIONS: "On-label" SGLT2 inhibitor use may increase DKA risk among insulin-treated T1D patients irrespective of sex, age, or BMI. Both T1D patients and healthcare providers should be wary of DKA, especially during the first month of initiating SGLT2 inhibitors.