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Objective: Exogenous insulin remains the leading therapy for type 1 diabetes mellitus, T1D. However, current insulin formulations and delivery methods do not match the precision of functioning beta-cells, and even intensive monitoring of blood glucose and use of insulin pumps are not enough to halt frequent hypoglycemia, hyperglycemia and morbidity. Thus, regenerating beta-cells has become a major target of diabetes research to treat T1D. While the hormone ghrelin is well known to increase blood glucose, at least in part by inhibiting insulin secretion, ghrelin’s action on beta-cell mass and proliferation has heretofore not been explored in detail. Here, we sought to test the hypothesis that reducing ghrelin levels increases beta-cell mass. Methods: Using histologic methods, we compared islet anatomy in ghrelin-knockout (ghrelin-KO) mice vs. wildtype littermates at three different ages: newborns (P0-P2) , juveniles (4-week-old) , and adults (10-12-week-old) . We also compared islet anatomy in adult wildtype mice, 4 weeks following conditional ghrelin-cell ablation vs. wildtype mice with intact ghrelin-cells. n= 4-8 mice per group and 33-433 islets per mouse. Results: Ghrelin gene deletion or ghrelin-cell ablation increased mean islet area and mean islet insulin-immunoreactive (insulin-IR) area by 52-82 % and by 45-51 %, respectively, in juveniles and adults. Increased islet size was driven by an increased % of large islets. Increased insulin-IR area was associated with increased beta-cell number per islet. An increased % of large islets was also observed in newborn ghrelin-KO mice. Conclusions: In mice, ghrelin plays an important role in controlling beta-cell mass. Reducing ghrelin levels for even short periods can increase islet size by increasing beta-cell numbers. Disclosure D.Gupta: None. C.P.Richard: None. J.M.Zigman: Research Support; Novo Nordisk Research Center Indianapolis, Novo Nordisk Research Center Indianapolis, Stock/Shareholder; Medtronic, Medtronic. A.W.Burstein: None. K.Shankar: None. S.Varshney: None. O.Singh: None. S.Paul: None. S.B.Ogden: None. S.Osborne-lawrence: None. N.Metzger: None.