Abstract Background Context Adult spinal deformity (ASD) represents a constellation of complex mal-alignments affecting the spinal column. Corrective surgical procedures aimed at improving ASD can be equally challenging, and commonly require multiple index procedures and potential revisions prior to definitive management. There is a paucity of data comparing the outcomes of same-day (simultaneous) and two-day (staged) procedures for long spinal-fusions for ASD. Utilizing a large patient cohort with surgeon and patient-reported outcomes will be particularly useful in determining the utility and effect of staging long spine fusions for ASD. Purpose Compare intra-operative, peri-operative, and two-year outcomes of staged and simultaneous procedures correcting ASD. Study Design/Setting Retrospective analysis of a prospective multi-center database. Patient Sample 142 patients (71 Staged, 71 Simultaneous). Outcome Measures Primary: intra- and peri-op (6 wk) complication rates. Secondary: 2 year thoracolumbar and spino-pelvic radiographic parameters, 2 year Health Related Quality of Life changes (Oswestry Disability Index and SF-36), and 2 year complication rates Methods Inclusion criteria included ASD patients ≥18yrs with 6-wk and 2 year follow-up. Propensity score matching identified similar patients undergoing staged (STA) or simultaneous (SIM) long spine fusions based on Surgical Invasiveness, Pelvic Tilt, and SVA. Complications, HRQLs (SRS22r, SF-36, ODI), and patient characteristics were compared across and within treatment groups at follow-up with ANOVA and paired t-tests at 3 surgical stages: intra-op, peri-op (6wk), and post-op (>6wk). Results 142 patients were included (71 STA, 71 SIM). Matching staged and simultaneous groups based on degree of deformity and surgical invasiveness created two groups similar in overall correction of the surgery. STA patients underwent more ALIF and LLIF interbody procedures while SIM patients had longer fusions. Charlson Comorbidity Index and revision status were similar between groups (p>0.05). There were significantly more complications causing reoperation in STA procedures (STA: 47% SIM: 8%, p=0.021). STA had a greater number of peri-op complications requiring a return to the OR (STA: 9.9% SIM: 1.4% p=0.029). There was no difference in intra-op complications, mortality, or peri-op infection or wound complications (p>0.05). At 2 year follow-up, incidence of revision surgery was higher in STA (STA: 21.1% SIM: 8.5%, p=0.033). Conclusion Staged spinal fusions which add ALIFs and LLIFs to the procedure, compared to similar-correction simultaneous procedures, result in similar intra-operative complication incidence, but significantly higher rates of peri- and post-op complications leading to revision. Functional outcomes, radiographic parameters, and mortality were similar. This will aid surgeons in their determination of optimal treatment for such complex procedures.