Islet transplantation is successful in animals and holds considerable promise as endocrine replacement therapy for patients with diabetes mellitus, but clinical application to diabetic patients has been difficult. We have shown the technical feasibility of human islet transplantation by autotransplantation of dispersed pancreatic islet tissue into the portal vein in three patients with chronic pancreatitis and incapacitating, intractable pain who underwent near-total (>97%) pancreatectomy. In all three patients, the excised pancreas was dispersed by collagenase digestion, but no effort was made to purify the islets. Islet yield, as judged by tissue insulin content, ranged from 24 to 55%. The first patient, who never received insulin after the pancreatectomy and islet autotransplantation, had a normal oral glucose tolerance test by 3 wk and has remained normoglycemic for over 2 yr. In the second patient, viable islets were histologically identified in the liver parenchyma. The third patient was treated with hyperalimentation for 3 wk after the pancreatectomy and islet autotransplantation and, during this period, required insulin. After cessation of hyperalimentation and initiation of oral feedings, the patient was withdrawn from insulin. Although abnormalities of carbohydrate metabolism were present, the patient did not require insulin for more than 1 yr. Seven diabetic renal allograft recipients have received allografts of dispersed pancreatic islet tissue prepared in the same way. No patients were cured of diabetes, although transient evidence of islet function - increase in serum or urinary C-peptide levels or decrease in exogenous insulin requirements - occurred in some. Although rejection was probably responsible for most of the failures, transplantation of allogeneic human islet tissue as a free graft is metabolically inefficient. With the current state of immunosuppressive therapy, the primary role of islet transplantation may be in a situation where rejection cannot occur: as an autograft to obviate the occurrence of diabetes after extensive pancreatectomy for benign disease.