This study describes unanticipated interstage readmissions in patients with hypoplastic left heart syndrome, identifies independent risk factors for unanticipated interstage readmissions, and evaluates variation in unanticipated readmission rates among collaborative centers. Retrospective data of patients enrolled in the National Pediatric Cardiology Quality Improvement Collaborative registry from July 2008 to July 2013 were analyzed. Risk factors present at the beginning of the interstage were captured. Competing risks time to event analyses determined the association between these factors and unanticipated interstage readmission. Readmission center variation was examined using funnel plots. Unanticipated interstage readmissions occurred in 66% of 815 patients at 50 centers. The median readmission length of stay was 2 days (interquartile range: 0-6) and median time to first readmission was 29 days (interquartile range: 9-63). Most readmissions were prompted by minor changes in clinical status (64%), whereas only 6% were major adverse event readmissions. Independent readmission risk factors included genetic syndrome (HR = 1.40, 95% CI: 1.05-1.88), center volume (small vs large HR = 1.32, CI: 1.04-1.66, medium vs large HR = 1.35, CI: 1.09-1.68), preoperative ventricular dysfunction (HR = 2.02, CI: 1.31-3.10), tricuspid regurgitation (HR = 1.36, CI: 1.08-1.72), duration of circulatory arrest (HR = 0.99, CI: 0.989-0.998), and undergoing Hybrid procedure relative to Norwood/right ventricle to pulmonary artery conduit (HR = 1.40, CI: 1.02-1.93). There was significant center variation in the number of readmissions and duration of readmissions. Unanticipated readmissions are common during the interstage period with notable center variation. However, these readmissions are short and are rarely in response to major adverse events.