Oral glucose tolerance test (OGTT)-related hypoglycemia is common in pancreatic insufficient cystic fibrosis (PI-CF), but its mechanistic underpinnings are yet to be established.To delineate the mechanism(s) underlying OGTT-related hypoglycemia.We performed 180-minute OGTT with frequent blood sampling in adolescents and young adults with PI-CF and compared results to those from a historical healthy control group. Hypoglycemia (Hypo[+]) was defined as plasma glucose <65 mg/dL. We hypothesized that CF-Hypo[+] would demonstrate impaired early-phase insulin secretion and persistent late insulin effect compared to Control-Hypo[+], and explored the contextual counterregulatory response.OGTT 1-hour and nadir glucose, insulin, C-peptide, and insulin secretory rate (ISR) incremental areas-under-the-curve (AUC) between 0-30 min (early) and 120-180 min (late), and Δglucagon120-180min and ΔFree Fatty Acids(FFA)120-180min were compared between CF- and Control-Hypo[+].Hypoglycemia occurred in 15/23 (65%) CF (43% female, age 24.8 [14.6-30.6] years) and 8/15 (55%) controls (33% female, age 26 [21-38] years). CF-Hypo[+] vs Control-Hypo[+] had higher 1-hour glucose (197±49 vs 139±53 mg/dL, p=0.05) and lower nadir glucose (48±7 vs 59±4 mg/dL, p<0.01), while insulin-, C-peptide-,and ISR-AUC0-30 min were lower and insulin- and C-peptide-AUC120-180min were higher (p<0.05). CF-Hypo[+] had lower Δglucagon120-180min and ΔFFA120-180min compared to Control-Hypo[+] (p<0.01).OGTT-related hypoglycemia in PI-CF is associated with elevated 1-hour glucose, impaired early-phase insulin secretion, higher late insulin exposure, and less increase in glucagon and FFA. These data suggest that hypoglycemia in CF is a manifestation of islet dysfunction including an impaired counterregulatory response.