Introduction: Hypoplastic left heart syndrome (HLHS) with restrictive atrial septum (RAS) is associated with poor outcomes. A subset of patients with HLHS have cor-triatriatum sinister, which often presents as a complex network of folds within the LA, which we referred to as LA-labyrinth (Figure 1). Hypothesis: We hypothesize that LA labyrinth is associated with worse clinical outcomes among those with a RAS compared to patients without LA-labyrinth. Methods: In a single-center retrospective cohort study, all fetuses with HLHS (2010-2020) were included. Fetuses with inadequate images, no follow-up data, pregnancy termination, or postnatal comfort care were excluded. All fetal echocardiograms were reviewed. RAS was defined as a ratio of forward/reverse pulmonary vein velocity time integral (PV-VTI) < 5. Kaplan-Meier analysis from 20 weeks’ gestation was performed to evaluate freedom from the primary outcome (fetal demise, postnatal death, or transplant) by 1-year postnatal life (72 weeks total). Results: Of 241 fetuses with HLHS, 179 met inclusion criteria. RAS was present in 43 (24%), and LA-labyrinth was seen in 9 (5%), of which 6 also had RAS. Freedom from death/transplant was worse in RAS with LA labyrinth vs. no LA labyrinth and vs. HLHS-unrestrictive atrial septum (Figure 2). Among those with RAS, those with LA labyrinth were more likely to have early atrial septal intervention (4/6 vs 10/34), and to die or need transplant (4/6 vs 12/37, p = 0.022). Conclusions: LA-labyrinth is seen in HLHS with RAS and is associated with worse outcomes than RAS alone. The unique left atrial anatomy may need to be taken into consideration during prenatal counseling and postnatal planning.