One endpoint of clinical islet cell transplantation for type 1 diabetic patients is the elimination or reduction of hypoglycemia. We previously developed a simple tool to evaluate islet graft function: the secretory unit of islet transplant objects (SUITO) index. The aim of this study is to clarify the association between the SUITO index and hypoglycemic episodes. Data from 310 clinical evaluations of 11 islet recipients were included in this study. Fasting plasma C-peptide and glucose levels were measured at every evaluation. The SUITO index was calculated according to the following formula: 1500 × C-peptide level (ng/ml)/[blood glucose level (mg/dl) - 63]. The number of hypoglycemic events (<3.8 mmol/L) and severe hypoglycemic events (<2.2 mmol/L or hypoglycemic unawareness) was assessed on the basis of interviews and self-monitoring of blood glucose (SMBG). Receiver operating characteristic (ROC) analysis was performed to determine the cut-off values of the SUITO index for hypoglycemic events. Based on the ROC study, follow-up data after transplantations were divided into the following three groups: low-SUITO (SUITO index <10, n = 91), middle-SUITO (10 ≤SUITO index <26, n = 83), high-SUITO (SUITO index ≤26, n = 125). The frequency of total hypoglycemia in the high-SUITO group was significantly decreased when compared to the other groups (value with Kruskal-Wallis test p < 0.001). The frequency of total severe hypoglycemia was significantly decreased in the low-SUITO group compared to pretransplant status and further decreased in the middle- and high-SUITO group. Spearman correlation coefficients were -0.663 (p < 0.001) between the number of total hypoglycemic events per one month and the SUITO index and -0.521 (p < 0.001) between that of severe events and the SUITO index. The SUITO index could predict the severity of hypoglycemic episodes in type 1 diabetic patients who received islet cell transplantations.