Study DesignRetrospective cross-sectional survey. ObjectiveTo investigate the timing of complications after posterior spinal fusion for idiopathic and neuromuscular pediatric spine deformity. Summary of Background DataEvidence is limited with regard to when complications occur after posterior spinal fusions in pediatric spine deformities. MethodsThe 2012–2016 American College of Surgeons–National Surgical Quality Improvement Program (ACS-NSQIP) Pediatric database files were queried using Current Procedural Terminology codes (22800, 22802, and 22804) for patients undergoing posterior spinal fusion for idiopathic or neuromuscular deformity. Median day-of-diagnosis and interquartile ranges were calculated for database-recorded complications. ResultsA total of 10,579 patients were included in the study. The frequency and median day of diagnosis of each complication are as follows: superficial surgical site infection (SSI) (idiopathic = 0.6%, Day 18.5; neuromuscular = 1.6%, Day 19.5), deep SSI (idiopathic = 0.5%, Day 16.0; neuromuscular = 2.3%, Day 18), organ/space SSI (idiopathic = 0.1%, Day 17; neuromuscular = 0.4%, Day 16), wound disruption (idiopathic = 0.4%, Day 15; neuromuscular = 1.2%, Day 15), pneumonia (idiopathic = 0.6%, Day 5; neuromuscular = 4.0%, Day 3), unplanned intubation (idiopathic = 0.4%, Day 2; neuromuscular = 3.5%, Day 1), urinary tract infection (idiopathic = 0.4%, Day 6; neuromuscular = 2.8%, Day 4.5), nerve injury causing neurologic deficit (idiopathic = 0.4%, Day 1; neuromuscular = 0.3%, Day 5), bleeding requiring transfusions (idiopathic = 64.6%, Day 0; neuromuscular = 74.1%, Day 0), sepsis (idiopathic = 0.3%, Day 11; neuromuscular = 2.4%, Day 12.5), and mortality (idiopathic = ∼0%, Day 5; neuromuscular = 0.4%, Day 9). ConclusionUnderstanding the timing of complications is important for patients and providers, as it reflects the need of heightened awareness and low thresholds of testing during periods of highest risks to catch complications, launch appropriate optimization protocols, and minimize the cost burden associated with readmissions. Level of EvidenceLevel IV.