Pancreatic islet transplantation as a treatment for type 1 diabetes is limited by human donor tissue availability. We investigated whether the beta-cell mass in human isolated islets could be expanded by treatments with glucagon-like peptide-1 (GLP-1) and gastrin, peptides reported to stimulate beta-cell growth in mice and rats with deficits in beta-cell mass. Human islets with low endocrine cell purity (7% beta-cells, 4% alpha-cells) and abundant exocrine cells (29% duct cells and 25% acinar cells) were implanted under the renal capsule of nonobese diabetic-severe combined immune deficiency (NOD-scid) mice made diabetic with streptozotocin. The mice were treated with GLP-1 and gastrin, separately and together, daily for 5 weeks. Blood glucose was significantly reduced only in mice implanted with human pancreatic cells and treated with GLP-1 plus gastrin. Correction of hyperglycemia was accompanied by increased insulin content in the human pancreatic cell grafts as well as by increased plasma levels of human C-peptide in the mice. Immunocytochemical examination revealed a fourfold increase in insulin-positive cells in the human pancreatic cell grafts in GLP-1 plus gastrin-treated mice, and most of this increase was accounted for by the appearance of cytokeratin 19-positive pancreatic duct cells expressing insulin. We conclude that combination therapy with GLP-1 and gastrin expands the beta-cell mass in human islets implanted in immunodeficient diabetic mice, largely from pancreatic duct cells associated with the islets, and this is sufficient to ameliorate hyperglycemia in the mice.