Background: Prior studies in various medical and surgical specialties have suggested that the changeover of medical trainees in the United States at the end of the academic year, or so-called "July effect," negatively impacts the quality of patient care, including increasing morbidity and decreasing efficiency. We analyzed whether the outcomes of surgery for adolescent idiopathic scoliosis involving physicians-in-training as first assistants were affected by the time of year the surgery was performed. Methods: We performed amulticenter retrospective study with use of a prospectively collected database to examine outcomes following instrumented posterior spinal fusion in patients with adolescent idiopathic scoliosis. Theminimum duration of follow-up was two years. The outcomes of procedures performed by twelve surgeons whose first assistants were all surgeons-in-training were analyzed on the basis of the month of year that the surgery was performed. Variables assessed included blood loss, operative time, length of hospitalization, radiographic outcomes, Scoliosis Research Society (SRS-22) scores, and complications. Results: Five hundred and seventy-five surgical procedures for adolescent idiopathic scoliosis were performed, most in June (14%) and July (13%) (p ≤ 0.001). Preoperative radiographic characteristics were similar across all months as were postoperative radiographic outcomes. Preoperative and two-year SRS-22 scores were also similar across all months, with the exception of scores in the preoperative pain domain, which showed worse pain for patients who underwent surgery in February. No significant differences in blood loss, operative time, or length of hospital stay were observed when these variables were analyzed on the basis of themonth inwhich the surgery was performed. The rate of patients experiencing any complication (23.5% overall) was not associated with themonth of surgery, nor were the rates for the specific subcategories of neurologic, pulmonary, gastrointestinal, instrumentation, or surgical site-related complications. With the exception of three gastrointestinal complications that were observed in July, the odds of a patient having a complication from surgery in July/August were unchanged from other months. Conclusions: Overall, the data did not provide evidence to support a July effect. Our results suggest that surgery for adolescent idiopathic scoliosis during July and August yields safety and outcomes equal to that of other months. Level of Evidence: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.