['In non-cystic fibrosis (CF) subjects, the disposition index (DI) is a strong predictor of the development of type 2 diabetes. CF subjects are at high risk of diabetes. We hypothesized that DI would be reduced in CF patients with normal glucose tolerance (NGT), indicating β-cell dysfunction, and DI would worsen with progression from CF with NGT to CF-related diabetes (CFRD).', 'This was a cross-sectional study in 39 CF patients and 21 healthy controls (Con) who underwent oral glucose tolerance test (OGTT). Insulin sensitivity was estimated as (1/fasting insulin) and insulin secretion as (∆insulin 0-30min/∆glucose 0-30min). DI was calculated as (insulin sensitivity)×(insulin secretion).', 'Among CF subjects, 14 had NGT, 20 had prediabetes and 5 had CFRD. Among the controls, 14 had NGT and 7 had prediabetes. DI was significantly lower in CF-NGT compared to Con-NGT (p=0.0035). DI was also lower in CFRD compared to CF-NGT (p=0.025). There were no significant relationships in the CF groups between DI and age, BMI, percent body fat or FEV1.'] Does disposition index identify defective beta-cell function in cystic fibrosis subjects with normal glucose tolerance?