Islet transplantation is a promising treatment for type 1 diabetes. However, islet grafts are submitted to multiple injuries, including immunosuppressive drug toxicity, hyperglycemia, hypoxia, unspecific inflammatory reactions, as well as allo-and autoimmune destruction. Therapeutic approaches to these damage mechanisms require early detection of islet injury, which is currently not feasible because of the lack of efficient markers. Based on the hypothesis of islet dissociation and release of islet cells into the circulation during islet injury, we designed a highly sensitive and specific molecular assay, able to detect two -cells per milliliter of venous blood by RT-PCR of insulin mRNA. We report that circulating -cells can be demonstrated up to 10 weeks after intraportal islet transplantation, as assessed after six islet grafts in four type 1 diabetic patients. Furthermore, our results suggest that the time during which circulating islet cells can be detected may depend on the graft environment and the immunosuppressive regimen. This test may allow better estimation of islet cell loss and identification of factors involved in islet graft injury. Diabetes 51:557-561, 2002 R eplacement of insulin-producing cells through islet transplantation is a promising approach for treatment of type 1 diabetes. This procedure is safe and relatively simple with rare complications, but, up to recently, only moderate success rates have been achieved at 1 year posttransplantation (with 41% graft survival, as defined by C-peptide Ն0.5 ng/ml, and 11% insulin independence) (1). The Edmonton protocol, including an immunosuppressive strategy without steroids, presently leads to 100% graft survival and 75% insulin independence in a median follow-up of 10.2 months (2,3).Whereas blood flow monitoring and amylase/lipase clearance (in case of bladder drainage), as well as serum amylase/lipase and nitric oxide concentrations, are used to asses pancreatic graft rejection (4), for islet transplantation, no early and reliable markers are available to monitor islet injury. Anti-GAD 65 antibodies are observed more frequently in patients experiencing an islet graft loss (5), but no systematic correlation can be found. Similarly, monitoring of serum GAD 65 levels could not be used as a marker of acute islet cell destruction (6). At present, islet graft survival is controlled by monitoring of glycemia, serum C-peptide, and HbA 1c levels; however, their sensitivity is low, and onset of hyperglycemia and C-peptide levels Ͻ0.5 ng/ml are end-stage events indicating the loss of Ͼ85% of the transplant (7,8). Evaluation of the functional reserve of transplanted islets by glucose tolerance tests or calculation of the proinsulin-to-insulin ratio seems to be more indicative for islet dysfunction, but still not sensitive enough for an early detection of graft loss (6,7). Our aim is to develop a reliable and early marker of the rejection process as well as a diagnostic tool for islet engraftment. As a first study, we set up a highly sensitive and specific m...