The aim of this study was to evaluate the feasibility of islet allografts in patients with type 1 diabetes mellitus. Six patients received human islets from either one or two donors via the portal vein, after (n = 4) or simultaneously with (n = 2) a kidney graft. The patients with functioning kidney grafts (nos. 1-4) were already on triple immunosuppressive therapy (cyclosporine A, azathioprine, prednisone). Prednisone was increased to 60 mg/day for 15 days after the islet transplant in patient 1. Patients 2-4 and the patients who underwent a simultaneous kidney-islets graft (nos. 5, 6) also received antilymphocyte globulin. Intravenous insulin was given for the first 15 days to maintain blood glucose concentrations within the normal range. Patient 1 rejected the islets within 15 days of islet transplantation. In patient 2, a 25% reduction in insulin requirement was observed and 12 months after transplantation post-prandial serum C-peptide was 1.5 ng/ml. In patient 3, the insulin requirement decreased from 40 to 8 units/day with a post-prandial serum C-peptide of 4.1 ng/ml 12 months after islet transplantation. In patient 4 the post-prandial secretion of C-peptide increased to 6.4 ng/ml. Six months after the islet infusion, insulin therapy was discontinued and HbA1c, 24-h metabolic profile and oral glucose tolerance test remained within the normal range. He had remained off insulin for 5 months until recently, when foot gangrene paralleled a worsening of post-prandial glycaemic control. Twelve months after transplantation he is receiving 8 units insulin/day.(ABSTRACT TRUNCATED AT 250 WORDS)