Evaluation of engrafted islets mass is important for clinical care of patients after islet transplantation. Recently, we developed the secretory unit of islet transplant objects (SUITO) index, which reflected engrafted islet mass. In this study, we evaluated the SUITO index for the prediction of clinical outcome after single islet transplantation. Single islet transplantations were performed into six type 1 diabetic patients. Isolated islets were quantitatively assessed at the time of transplantation. The SUITO index was calculated as follows: fasting C-peptide (ng/dl)/[fasting blood glucose (mg/dl) - 63] x 1500. Islet yield/recipient's body weight and SUITO index were evaluated, along with HbA(1C), relative insulin dose (insulin dose posttransplant/pretransplant), and M-values. HbA(1C) improved in all cases, irrespective of the SUITO index score or islet yield/body weight. The average SUITO index from postoperative days 3 to 30 (R(2) = 0.728, p < 0.04), but not islet yield/body weight (R(2) = 0.259, p = 0.303), correlated with relative insulin dose. The daily SUITO index strongly correlated with the daily relative insulin dose (R(2) = 0.558, p < 0.0001) and weakly correlated with the daily M-values (R(2) = 0.207, p < 0.02). A SUITO index score of less than 10 was associated with increasing insulin dose even after islet transplantation. The SUITO index seems to be a better predictor of success of islet transplantations than islet yield/body weight. SUITO index is recommended to assess clinical outcome of islet transplantation.