<h3>BACKGROUND CONTEXT</h3> The lumbosacral fractional curve is often more challenging to correct vs the thoracolumbar/lumbar (TL/L) curve during adult spinal deformity (ASD) surgery, with implications for postoperative coronal alignment. <h3>PURPOSE</h3> In patients undergoing ASD surgery, the lumbosacral fractional (LSF) curve significantly impacts preop & postop coronal malalignment (CM). <h3>STUDY DESIGN/SETTING</h3> Retrospective cohort <h3>PATIENT SAMPLE</h3> A total of 243 adult spine deformity patients. <h3>OUTCOME MEASURES</h3> Postoperative lumbosacral fractional and maximum Cobb curve correction, coronal vertical axis. <h3>METHODS</h3> Patients undergoing ≥6 level ASD surgery at a single-institution were collected. The LSF curve was the Cobb angle between the sacrum & most tilted lower lumbar vertebra. Coronal vertical axis (CVA) from C7-mid sacrum (cm) was collected. Patients were grouped by alignment: 1) Neutral alignment (N): CVA≤3cm & SVA≤5cm, 2) Coronal malalignment only (CM): CVA>3cm, 3) Sagittal malalignment only (SM): SVA>5cm, or 4) Coronal and sagittal malalignment (CCSM): both CVA>3cm & SVA>5cm. <h3>RESULTS</h3> A total of 243 patients underwent ASD surgery: N 115 (47.3%), CM 48 (19.8%), SM 38 (15.6%), CCSM 42 (17.6%). Mean LSF curve was 12.1±9.9° (0.2-62.3). An LSF curve >5° was seen in 174 (71.6%) of all patients & 81.1% of patients with preoperative CM (p=0.012). Largest LSF curves were seen in the CM (14.6±11.9°) & CCSM (13.1±8.3°) groups. 83% of patients had the LSF curve opposite the max Cobb angle. A significant linear relationship was seen between preop LSF curve & preoperative CVA (β =0.03, p=0.042). Immediately postoperative, a linear relationship was seen between CVA and the postop LSF curve (r=0.147, p=0.022), as well as CVA & max Cobb angle (r=0.148, p=0.021), indicating that postoperative coronal alignment is correlated to how much each curve is corrected. Max coronal Cobb angle achieved more % correction than LSF curve (54.5% Cobb vs 46.5% LSF, p=0.025). Postop CM was significantly associated with both the LSF curve to the same side as the CVA (OR 2.3, 95%CI 1.14-4.68, p=0.021) and the max coronal Cobb opposite the CVA (OR 2.1, 95%CI 1.1-4.2, p=0.033), with LSF curve directionality being the stronger predictor. <h3>CONCLUSIONS</h3> A lumbosacral fractional curve >5° was seen in 81% of adult spinal deformity patients with preoperative coronal malalignment. The lumbosacral fractional curve opposed the max Cobb angle in 83% of cases. The lumbosacral fractional curve was a slightly stronger driver of postop CM than max Cobb angle, potentially due to it being corrected less or not included in the surgery as compared to the max Cobb angle. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.