OBJECTIVES To estimate medication noninitiation prevalence in the pediatric population and identify the explanatory factors underlying this behavior. METHODS Observational study of patients (<18 years old) receiving at least 1 new prescription (28 pharmaceutical subgroups; July 2017 to June 2018) in Catalonia, Spain. A prescription was considered new when there was no prescription for the same pharmaceutical subgroup in the previous 6 months. Noninitiation occurred when a prescription was not filled within 1 month or 6 months (sensitivity analysis). Prevalence was estimated as the proportion of total prescriptions not initiated. To identify explanatory factors, a multivariable multilevel logistic regression model was used, and adjusted odds ratios were reported. RESULTS Overall, 1 539 003 new prescriptions were issued to 715 895 children. The overall prevalence of 1-month noninitiation was 9.0% (ranging from 2.6% [oral antibiotics] to 21.5% [proton pump inhibitors]), and the prevalence of 6-month noninitiation was 8.5%. Noninitiation was higher in the youngest and oldest population groups, in children from families with a 0% copayment rate (vulnerable populations) and those with conditions from external causes. Out-of-pocket costs of drugs increased the odds of noninitiation. The odds of noninitiation were lower when the prescription was issued by a pediatrician (compared with a primary or secondary care clinician). CONCLUSIONS The prevalence of noninitiation of medical treatments in pediatrics is high and varies according to patients’ ages and medical groups. Results suggest that there are inequities in access to pharmacologic treatments in this population that must be taken into account by health care planners and providers.