Ex vivo expansion of human β-cells is an important step toward the development of cell-based insulin delivery systems in type 1 diabetes. Here, we report that human pancreatic endocrine cells can be expanded through 15 cell doublings in vitro for an estimated total 30,000-fold increase in cell number. We believe that the cells resulting from these cultures are of β- cell origin, since they uniformly express the transcription factor PDX-1 (STF-1, IDX-1, IPF-1), which is initially seen only in cells positive for insulin and negative for the ductal cell marker cytokeratin (CK)-19. To rule out the possibility that PDX-1 expression might be induced by the culture conditions used here, cells from isolated human pancreatic ducts were cultured under the same conditions as the islet cells. Cells in these cultures expressed CK-19 but not PDX-1. Although the expanded β-cells continued to express PDX-1, insulin expression was lost over time. Whether reexpression of islet-specific genes in vitro is essential for successful cell transplantation remains to be determined.