Evidence is accumulating that prolonged fasting (PL) might evoke improvements in metabolic health. Since fasting is accompanied by numerous endogenous processes to equilibrate blood glucose concentration, its feasibility is disputable in people with type 1 diabetes (PWT1D). We aimed to assess the safety and efficacy of PL in c-peptide negative (^< 0.9 ng/mL) PWT1D. Methods: Glucose metabolism was assessed during the fasting period (CGM) and via a 75 g OGTT accompanied with the same bolus insulin dose after 12 (shortF) and 36 hours of fasting (longF). Area under the curve (AUC) was calculated following the trapezoidal rule. Safety was assessed by the number of hypoglycemic episodes (< 70 mg/dL) during fasting. Data were analysed using a paired sample t-test or Mann-Whitney-U-test (p≤0.05). Results: In total, 17 PWT1D (6 women, aged 37 ± 11 years, BMI 24.9 ± 2.9 kg/m2, 10 MDI) were included. 13 episodes of mild hypoglycemia occurred in 9 participants during the shortF versus 27 in 15 participants during the longF. Incidence of hypoglycemia per hour was similar in both arms (median [IQR] shortF: 0.08 [0 - 0.08] vs. longF: 0.06 [0.03 - 0.07], p=0.62). Conclusion: PL did not increase the risk of hypoglycemia in PWT1D. Intriguingly, the AUC for glucagon was significantly higher after longF compared to shortF, potentially explaining the numerically lower number of hypoglycemic events during longF.