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Automated insulin delivery systems improve blood glucose control in patients with type 1 diabetes (T1D). However, optimizing their performance requires patient's proper compliance to meal insulin bolus administration. We explored real-life prevalence of delayed prandial boluses (DBs) in adults with T1D on advanced technologies, and their association with glycemic control and fear of hypoglycemia (FH). In the last two-week web-based reports of 152 adults with T1D on Hybrid Closed Loop Systems (HCLS) or Sensor Augmented Pump (SAP), DBs were identified when a steep increase in blood glucose occurred at CGM before the prandial bolus, and CGM metrics were evaluated. All participants completed an online questionnaire on FH. Mean DBs over two weeks were 10.2 ± 4.7 (M ± SD, range 1–23) and more frequent in women than men (11.0 ± 4.6 vs. 9.4 ± 4.7, p = 0.036). Participants with more DBs (>12) showed significantly lower Time-In-Range (62.4 ± 13.8 vs. 76.6 ± 9.0 %) than those with less DBs (<7.7), along with higher Time-Above-Range, GMI, and Coefficient-of-Variation (ANOVA, p < 0.001 for all). Participants with higher FH score showed more DBs (11.6 ± 5.0) than those in lower tertiles (9.57 ± 4.59 and 9.47 ± 4.45, ANOVA p = 0.045). In patients on advanced technologies, delayed boluses are extremely common, and associate with significantly worse glycemic control. Utmost attention is needed to bolus timing, mainly tackling fear of hypoglycemia. • Prandial insulin boluses are frequently delayed in patients with type 1 diabetes. • The delay impairs blood glucose control also in patients on advanced technologies. • Delayed insulin boluses are often related to fear of hypoglycemia. • Clinical care should focus on this issue, including appropriate attention to fear of hypoglycemia. [ABSTRACT FROM AUTHOR]