BACKGROUND CONTEXT: Significant variability in neurologic outcomes after surgical correction for adult spinal deformity (ASD) has been reported. Risk factors for decline in neurologic motor outcomes are poorly understood. PURPOSE: The objective of the present investigation was to identify the risk factors for postoperative neurologic motor decline in patients undergoing complex ASD surgery. STUDY DESIGN/SETTING: This is a prospective international multicenter cohort study. PATIENT SAMPLE: From September 2011 to October 2012, 272 patients undergoing complex ASD surgery were prospectively enrolled in a multicenter, international cohort study in 15 sites. OUTCOME MEASURES: Neurologic decline was defined as any postoperative deterioration in American Spinal Injury Association lower extremity motor score (LEMS) compared with preoperative status. METHODS: To identify risk factors, 10 candidate variables were selected for univariable analysis from the dataset based on clinical relevance, and a multivariable logistic regression analysis was used with backward stepwise selection. RESULTS: Complete datasets on 265 patients were available for analysis and 61 (23%) patients showed a decline in LEMS at discharge. Univariable analysis showed that the key factors associated with postoperative neurologic deterioration included older age, lumbar-level osteotomy, three-column osteotomy, and larger blood loss. Multivariable analysis revealed that older age (odds ratio [OR]=1.5 per 10 years, 95% confidence interval [CI] 1.1-2.1, p=.005), larger coronal deformity angular ratio [DAR] (OR=1.1 per 1 unit, 95% CI 1.0-1.2, p=.037), and lumbar osteotomy (OR=3.3, 95% CI 1.2-9.2, p=.022) were the three major predictors of neurologic decline. CONCLUSIONS: Twenty-three percent of patients undergoing complex ASD surgery experienced a postoperative neurologic decline. Age, coronal DAR, and lumbar osteotomy were identified as the key contributing factors.