The humoral immune response to islet autoantigens, here defined by the presence of islet cell antibodies (ICA) and glutamic acid decarboxylase (GAD 65) antibodies, was studied in patients with long-term insulin-dependent diabetes mellitus (IDDM) receiving immunosuppressive therapy following kidney and islet-after-kidney transplantation. In a cross-sectional study of 30 kidney-grafted, long-term IDDM patients and 30 matched, nontransplanted IDDM controls, we observed a significant (P<0.05) decrease in ICA positivity by standard immunosuppressive therapy, but not in frequency or index levels of GAD 65 antibodies. Because of this intriguing finding, we investigated, in a pilot study on seven islet-after-kidney transplant recipients, the time course of frequency and levels of ICAs and GAD 65 antibodies relative to islet graft function. Stable islet graft function was seen in the patients with low GAD 65 antibody index levels, whereas rapid islet graft failure occurred in a patient with high GAD 65 antibody index levels prior to transplantation. In addition, GAD 65 autoimmunity reoccurred in one pretransplant antibody-negative patient 2 months after graft failure was noted. In conclusion, these observations suggest that β-cell autoimmunity directed to GAD 65 can persist despite immunosuppressive therapy and may adversely affect islet graft function, possibly indicating disease recurrence as a major threat to successful clinical islet transplantation.