BACKGROUND AND OBJECTIVES: Filling a prescription is the important first step in medication adherence, but has not been studied in pediatric primary care. The objective of this study was to use claims data to determine the rate of unfilled prescriptions in pediatric primary care and examine factors associated with prescription filling. METHODS: This retrospective observational study of pediatric primary care patients compares prescription data from an electronic medical record with insurance claims data. Illinois Medicaid provided claims data for 4833 patients who received 16 953 prescriptions during visits at 2 primary care sites over 26 months. Prescriptions were compared with claims to determine filling within 1 day and 60 days. Clinical and demographic variables significant in univariate analysis were included in logistic regression models. RESULTS: Patients were 51% male; most (84%) spoke English and were African American (38.7%) or Hispanic (39.1%). Seventy-eight percent of all prescriptions were filled. Among filled prescriptions, 69% were filled within 1 day. African American, Hispanic, and male patients were significantly more likely to have filled prescriptions. Younger age was associated with filling within 1 day but not with filling within 60 days. Prescriptions for antibiotics, from one of the clinic sites, from sick/follow-up visits, and electronic prescriptions were significantly more likely to be filled. CONCLUSIONS: More than 20% of prescriptions in a pediatric primary care setting were never filled. The significant associations with clinical site, visit type, and electronic prescribing suggest system-level factors that affect prescription filling. Development of interventions to increase adherence should account for the factors that affect primary adherence.