Objective: With improving operative mortality for staged palliation of hypoplastic left heart syndrome, interstage death accounts for an increasing proportion of hypoplastic left heart syndrome mortality. We investigated risk factors for death or cardiac transplantation during the interstage period between bidirectional Glenn and Fontan procedures in children with hypoplastic left heart syndrome. Methods: Patients with hypoplastic left heart syndrome who underwent bidirectional Glenn between August 1995 and June 2007 were screened. Standard risk patients, defined by having been discharged after both Norwood and bidirectional Glenn, were included for analysis. Patient demographic, echocardiographic, cardiac catheterization, and operative data were reviewed. Interstage attrition was defined as death or cardiac transplantation more than 30 days after bidirectional Glenn and before the Fontan procedure. Statistical analysis was carried out using the Student t test, Pearson chi-square correlation, and Cox proportional hazard modeling for multivariable analysis. Results: Ninety-two patients with hypoplastic left heart syndrome were alive at 30 days after bidirectional Glenn. Of these patients, 8 died and 3 underwent cardiac transplantation at a median of 391 days (range, 59-1175 days) after bidirectional Glenn, yielding an interstage attrition rate of 12%. Removing the 7 patients who are still awaiting Fontan (but all of whom are at least 3.5 years after bidirectional Glenn) adjusts the attrition rate to 13%. Interstage attrition did not correlate with hemodynamic data obtained at cardiac catheterization, aortic arch obstruction, or right ventricular dysfunction. Multivariable analysis demonstrated that the presence of moderate or severe tricuspid valve regurgitation (hazard ratio, 6.02; 95% confidence interval, 1.56-23.24; P < .01) and weight z score (hazard ratio, 0.38; 95% confidence interval, 0.16-0.88; P = .02) were independent preoperative risk factors for interstage attrition. Conclusions: Interstage attrition between bidirectional Glenn and Fontan procedures occurred in 12% of our study population. Moderate or greater tricuspid valve regurgitation and low weight z score at the time of bidirectional Glenn are important risk factors for interstage attrition between the bidirectional Glenn and Fontan procedures in children with hypoplastic left heart syndrome. Copyright © 2011 by The American Association for Thoracic Surgery.