Recent advances in human islet transplantation have highlighted the need for expanding the pool of β-cells available for transplantation. We have developed three transgenic models in which growth factors (hepatocyte growth factor [HGF], placental lactogen, or parathyroid hormone-related protein) have been targeted to the β-cell using rat insulin promoter (RIP). Each displays an increase in islet size and islet number, and each displays insulin-mediated hypoglycemia. Of these three models, the RIP-HGF mouse displays the least impressive phenotype under basal conditions. In this study, we show that this mild basal phenotype is misleading and that RIP-HGF mice have a unique and salutary phenotype. Compared with normal islets, RIP-HGF islets contain more insulin per β-cell (50 ± 5 vs. 78 ± 9 ng/islet equivalent [IE] in normal vs. RIP-HGF islets, P < 0.025), secrete more insulin in response to glucose in vivo (0.66 ± 0.06 vs. 0.91 ± 0.10 ng/ml in normal vs. RIP-HGF mice, P < 0.05) and in vitro (at 22.2 mmol/l glucose: 640 ± 120.1 vs. 1,615 ± 196.9 pg · μg protein⁻¹ · 30 min⁻¹ in normal vs. RIP-HGF islets, P < 0.01), have two- to threefold higher GLUT2 and glucokinase steady-state mRNA levels, take up and metabolize glucose more effectively, and most importantly, function at least twice as effectively after transplantation. These findings indicate that HGF has surprisingly positive effects on β-cell mitogenesis, glucose sensing, β-cell markers of differentiation, and transplant survival. It appears to have a unique and unanticipated effective profile as an islet mass- and function-enhancing agent in vivo.