The current study was planned to evaluate deformity characteristics, assess relationship between morphology of syrinx/ Arnold Chiari malformation (ACM) and deformity, analyze effect of posterior fossa decompression (PFD), and evaluate outcome. Overview of Literature: Scoliosis in ACM-I and syringomyelia (SM) is uncommon, and deformity characteristics differ from those seen in idiopathic scoliosis. Methods: Data regarding patients, who underwent PFD for ACM-I presenting with SM and scoliosis between January 2009 and December 2018, were retrospectively collected. Only patients with 2-year follow-up were included. Sagittal/coronal deformity and sagittal spinopelvic parameters were examined. Symmetry and extent of tonsillar descent, as well as morphology (configuration/variation) and extent of syrinx were determined. Results: A total of 42 patients (20 females; age: 14.2±5.8 years) were included; 35 patients (83.3%) had atypical curves. Mean preoperative coronal Cobb was 57.7°±20.9°; and 12 (28.6%) had significant coronal imbalance. Tonsillar descent was classified as grade 1, 2, and 3 in 16 (38.1%), 11 (26.2%), and 15 (35.7%) patients; 35 patients (83.3%) had asymmetric tonsillar descent; 17 (40.4%), 3 (7.1%), 16 (38.1%), and 6 (14.4%) had circumscribed, moniliform, dilated, and slender syrinx patterns; and 9 (21.4%), 12 (28.6%), and 21 (50%) of syrinx were right-sided, left-sided, and centric. There was no significant relationship between side of tonsillar dominance (p=0.31), grade of descent (p=0.30), and convexity of deformity. There was significant association between side of syrinx and convexity of scoliosis (p=0.01). PFD was performed in all, and deformity correction was performed in 23 patients. In curves ≤40°, PFD alone could stabilize scoliosis progression (p=0.02). There was significant reduction in syrinx/cord ratio following PFD (p<0.001). Conclusions: ACM-I+SM patients had atypical curve patterns in 83% of cases, and the side of syrinx deviation correlates with scoliosis convexity. Syrinx shrinks significantly following PFD. PFD may not stabilize scoliosis in curves >40°.