BACKGROUND: Adherence to oral chemotherapy, including 6-mercaptopurine (6-MP), is suboptimal in pediatric acute lymphoblastic leukemia (ALL), which is associated with increased risk of relapse. Study objectives were to examine self-reported adherence to 6-MP and related barriers to adherence, mapped to the capability, opportunity, motivation, and behavior (COM-B) model for behavior change. PROCEDURE: Forty-nine parents (median, 39 years old; 76% females) and 15 patients (median, 17 years old, 20% females) completed the study survey. RESULTS: Suboptimal adherence was reported in 43% of parents and 73% of patients. Most parents and patients (80% and 90%, respectively) reported ≥1 adherence barrier. Parents reported difficulty helping their child meet others with ALL (43%), contacting community organizations (39%), and meeting other parents (37%). Patients reported difficulty finding out what their medications are (40%), finding out what 6-MP does (47%), and meeting other patients (40%). Using the COM-B, we found that parents and patients endorsed barriers in multiple components, especially physical (55%, 67%) and social opportunity (56%, 47%), highlighting that barriers to adherence may be multifaceted. CONCLUSIONS: Our results suggest that parents and patients with ALL face various prevalent barriers to medication adherence and provide insight into the development of behavioral interventions focused on promoting adherence, which is essential to prevent relapse and optimize health outcomes in ALL.