BACKGROUND: Although it is generally agreed upon that surgery for high-grade spondylolisthesis (HGS) is associated with more complications than low-grade spondylolisthesis, its description is primarily based on case reports and relatively small case series. OBJECTIVE: To assess short-term complication rates associated with the surgical treatment of HGS in pediatric and adult patients and to identify factors associated with increased complication rates. METHODS: All cases of HGS from the Scoliosis Research Society Morbidity and Mortality database for the year 2007 were reviewed. Patients were classified as pediatric (≤18 years) or adult (>18 years). Complications were tabulated, and the rates were compared between the patient groups and based on clinical and surgical factors. RESULTS: 165 cases of HGS were reported (88 pediatric, 77 adult). There were 49 complications (29.7%) in 41 patients (24.8%), with no difference in the proportions of pediatric vs adult patients with a complication (P = .86). Occurrence of new neurological deficit after surgery was the most common complication, seen in 19 (11.5%) patients. Performance of an osteotomy was associated with a higher incidence of new neurological deficits in both adult and pediatric groups (P = .02 and P = .012, respectively). Although most of the new neurological deficits improved over follow-up, 10% had no improvement. CONCLUSION: This study provides short-term complication rates associated with surgical treatment for HGS in adult and pediatric patients and may prove valuable for patient counseling, surgical planning, and in efforts to improve the safety of patient care.