The objective of this study was to determine the contemporary etiologies, treatment, and outcomes of moderate and large pericardial effusions in pediatric patients. We reviewed pediatric patients with moderate or large effusions diagnosed at Children's Hospital Boston. Effusion size was determined in offline review of echocardiograms. One hundred sixteen patients with moderate or large pericardial effusions were identified. The age range was 1 day to 17.8 years (median 8.6). The size of the pericardial effusions ranged from 0.5 to 4.7 cm (median 2.1). Neoplastic disease was present in 39% of patients, collagen vascular disease in 9%, renal disease in 8%, bacterial infection in 3%, and human immunodeficiency virus (HIV) in 2%; 37% were idiopathic. Pericardial drainage procedures were performed in 47 patients (41%). Of these, 29 (63%) had recurrent effusions leading to repeat drainage in 12 (41%). Pericardial effusions resolved within 3 months in 83% of patients who underwent drainage and in 91% of patients who did not. In summary, pediatric pericardial effusions were rarely caused by bacterial infections in this study population and were more frequently idiopathic or associated with neoplastic disease. Pericardial effusions often reaccumulated after drainage. The majority of both drained and undrained effusions resolved within 3 months.