While some studies suggest that a linear dose-response relationship exists between exercise and insulin sensitivity, the exercise dose required to enhance pancreatic beta-cell function is unknown. Thirty-five older, obese adults with prediabetes underwent a progressive 12-week supervised exercise intervention (5d/wk for 60min at ~85% HRmax). Insulin and C-peptide (n=23) responses to an OGTT were used to define the first and second phase disposition index (DI; beta-cell function = glucose-stimulated insulin secretion x clamp-derived insulin sensitivity). Maximum oxygen consumption (VO2max) and body composition (dual-energy x-ray absorptiometry and computed tomography) were also measured before and after the intervention. Exercise dose was computed using VO2-heart rate derived linear-regression equations. Subjects expended 474.5±8.8 kcal/session (2372.5±44.1 kcal/week) during the intervention, and lost ~8% body weight. Exercise increased first and second phase DI (P2000 kcal/week) are necessary to enhance beta-cell function in adults with poor insulin secretion capacity.