Background: This multicenter crossover study investigated the potential beneficial effect of an automated bolus calculator (ABC) in children and adolescents with type 1 diabetes (T1D) treated with multiple daily injections (MDI). Methods: Participants were randomized to either begin or end with a 5 months intervention versus their regular treatment regimen (control), separated by a 2 months washout period. During the intervention participants were carefully instructed to use the ABC (Accu-Check Aviva Expert) versus manual insulin calculations during the control period. Participants between 8 and 18 years of age with T1D were recruited from clinics in Denmark, Belgium, and Spain. Inclusion criteria included T1D for >1 year, a minimum of 3 months MDI treatment before inclusion, and HbA1c of 7.5%-11% (57-97 mmol/mol). Improvement in HbA1c was the main outcome, and improved quality of life (QoL) and glucose variability (time spent in target glucose) were secondary outcomes. Results: A total of 65 patients with a mean age of 13.25 years and a mean HbA1c of 8.25% (66.7 mmol/mol) were included. Midway evaluation after 2 months of intervention showed no significant difference from the standard care (0.297, 95% confidence interval [CI]: -0.645 to 0.054; P = 0.10). The difference remained insignificant after the 5 months of intervention (-0.143 [95% CI: -0.558 to 0.272; P = 0.51]). Using the ABC did not change the time spent in target glucose range, nor did it change the QoL. Conclusions: Our study did not demonstrate beneficial additive effects of an ABC in children and adolescents with T1D treated with MDI neither in HbA1c, nor in any other endpoint investigated.