A topic of current interest in islet transplantation is the selection of an optimal site for long-term graft survival since the intrahepatic site may be characterized by long-term failure. Additionally, the use of immunosuppressive agents such as prednisone may adversely affect long-term graft function. In this study, we examined the long-term outcome of intrahepatic canine islet autografts and compared this with results obtained in animals treated with a short-term course of steroids or steroids plus insulin. Islets were isolated using the automated method and were purified on discontinuous EuroCollins Ficoll gradients (densities: 1.108, 1.096, 1.037). Prednisone-treated dogs were hyperglycemic during treatment but returned to normoglycemia after steroid withdrawal. Control and insulin-treated animals were normoglycemic following autotransplant, with no difference in plasma glucose levels between controls and the insulin-treated animals. All control dogs became diabetic at 11,14,17, and 19 months following islet autograft. Prednisonetreated dogs had more rapid onset of diabetes at 7,11, and 12 months following ITx. Prednisone-treated dogs given insulin became hyperglycemic at 10, 14, 18, and 19 months post ITx. Graft failure was preceded by a decline in IVGTT Kg values and diminished insulin secretion. At the time of graft failure islets showed no lymphocytic infiltration and islets stained positive for glucagon but few insulin-containing cells were seen. Thus, even when an initially adequate B cell mass was transplanted, the intrahepatic site was characterized by long-term canine autograft failure. A short course of prednisone accelerated the time to graft failure and insulin treatment reversed this acceleration.