Objective To assess the efficacy and safety of insulin detemir administered once vs. twice daily in children and adolescents with type 1 diabetes mellitus. Research Design In this prospective, open-label, treat-to-target study, 37 patients [mean age 12.7 +/- 3yr; diabetes duration 4.2 +/- 3yr, hemoglobin A1c (HbA1c) 8.8 +/- 0.8%] were scheduled to receive insulin detemir once daily before breakfast, with pre-meal insulin aspart, for 1620wk. Detemir dose titration algorithm was based on age-related target fasting blood glucose levels during 48wk. Patients achieving target range continued on once-daily detemir (Group A) if up-titration could not be done due to hypoglycemia patients were switched to twice-daily detemir (Group B). Results Nineteen (51%) patients continued with once-daily detemir. HbA1c decreased significantly in both groups (A: 0.7%, p=0.02; B: 0.8%, p=0.004), without a significant difference between groups. The frequency of nocturnal hypoglycemic events/week decreased in both groups but a significant change was found only in Group A (10.92.7, p<0.05 vs. 8.75.8, NS), with no change in frequency of severe hypoglycemic episodes in either group. No significant differences were found between and within groups for body mass index-standard deviation score, insulin requirement or treatment satisfaction. Group B patients were significantly younger than Group A patients (11.5 +/- 2.3 vs.13.8 +/- 3.2yr, p=0.01), with a higher percentage in active puberty (50 vs. 11%, p=0.003). Conclusion Since twice-daily determir showed no clinical advantage over once-daily detemir, it appears reasonable to commence all children on once-daily detemir, taking into consideration that younger children and those in active puberty may require twice-daily therapy (ClinicalTrials.gov number, NCT00542399).