Aims Insulin degludec (IDeg) is a new-generation basal insulin with an ultra-long duration of action. We compared the pharmacodynamic (PD) variability of IDeg and insulin glargine (IGlar) under steady-state conditions. Methods Day-to-day variability in glucose-lowering effect was investigated in 54 subjects with type 1 diabetes who underwent a 24-h euglycaemic glucose clamp on the 6th, 9th and 12th day of treatment with 0.4 U/kg of IDeg or IGlar once daily. Within-subject variability was estimated using a linear mixed model on log-transformed PD endpoints derived from the glucose infusion rate (GIR) profiles during the clamps. Results For IDeg the day-to-day variability in glucose-lowering effect was four-times lower than for IGlar for total metabolic effect (AUCGIR,0-24h,SS, CV 20% vs. 82%) and for the last 22 h [AUCGIR,2-24h,SS (not influenced by intravenous insulin during the clamp), CV 22% vs. 92%]. Furthermore, lower variability in the maximum effect was observed for IDeg vs. IGlar (GIRmax,SS, CV 18% vs. 60%). The lower within-subject variability of IDeg was consistent over time (CVs of 33% for AUCGIR,0-2h,SS, 32% for AUCGIR,10-12h,SS and 33% for AUCGIR,22-24h,SS), whereas the variability of IGlar was higher and increased substantially 8 h post-dosing (CVs of 60% for AUCGIR,0-2h,SS, 135% for AUCGIR,10-12h,SS and 115% for AUCGIR,22-24h,SS). Conclusions These results show that IDeg has a significantly more predictable glucose-lowering effect from day to day than IGlar.