Insulin detemir (detemir) has previously been shown to be associated with lower within-subject variability compared with other basal insulin preparations in adults with type 1 diabetes mellitus (T1DM). This randomized, double-blind, crossover trial compared the within-subject variability of detemir and insulin glargine (glargine) in pharmacokinetic properties in children and adolescents with T1DM. The trial enrolled 32 children and adolescents (19 girls and 13 boys; mean ± SD: age 13 ± 2.5 yr and T1DM duration 6.3 ± 3.0 yr) with a hemoglobin A1c (HbA1c) of 7.9 ± 1.0%. Participants were randomized to a specific treatment sequence in which a dose of 0.4 U/kg of detemir and glargine was injected subcutaneously 24 h apart at each of two dosing visits. Insulin concentrations were measured at frequent intervals for a period of 16-h post-dosing. Detemir showed statistically significantly less within-subject variability compared with glargine with a 3.1-fold and 2.9-fold lower coefficient of variation (CV, %) for the area under the concentration–time curve [AUC(0–16h)] and the maximum concentration (Cmax), respectively. Separate analyses demonstrated a 2.5-fold and 2.9-fold lower CV (%) with detemir in children (8–12 yr) and a 4-fold and 3.8-fold lower CV (%) with detemir in adolescents (13–17 yr). No safety concerns were raised during the trial. In conclusion, within-subject variability in pharmacokinetic properties was significantly lower for detemir than for glargine in children and adolescents with T1DM. This indicates a less variable absorption with detemir, which is expected to be associated with a more predictable therapeutic effect also in this population.