AIMS: High glycemic variability (GV) is the major indication for islet transplantation (IT) in patients with type 1 diabetes (T1D). The actual criteria used to assess graft function do not consider GV improvement. Our study aimed to describe GV indices' evolution in T1D patients who benefited from IT during the TRIMECO trial and to evaluate if thresholds might be defined to diagnose IT success. METHODS: We collected data from 29 patients of the TRIMECO trial, a clinical trial (NCT01148680) comparing the metabolic efficacy of IT with intensive insulin therapy. Based on CGM data, we analyzed mean glucose level and four GV indices (standard deviation, coefficient of variation, MAGE and GVP) before (M0) and 6 months (M6) after IT. RESULTS: Each GV index decreased significantly between M0 and M6: SD 53.9 mg/dL [44.6-61.5] versus 20.1 mg/dL [13.5-24.3]; CV 35.2% [30.6-37.7] versus 17.3% [12.0-20.5]; MAGE 134.9 mg/dl [111.2-155.8] versus 51.9 mg/dL [32.4-62.4]; GVP 35.3% [24.9-47.2] versus 12.2% [6.2-18.8] (p </= 0.0001). Thresholds diagnosing IT success at 6 months post-transplant were an SD at 22.76 mg/dL (sensibility 88.89%, specificity 80.00%), a CV at 17.47% (sensibility 88.89%, specificity 70.00%), a MAGE at 54.81 mg/dL (sensibility 88.89%, specificity 80.00%) and a GVP at 12.27% (sensibility 88.89%, specificity 70.00%). CONCLUSIONS: This study confirms a positive impact of IT on GV. The proposed thresholds allow an easy evaluation of IT success using only CGM data and may be a clinical tool for the follow-up of transplanted patients.