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The modified Dunn procedure has rapidly gained popularity as a treatment for unstable slipped capital femoral epiphysis (SCFE), but limited data exist regarding its safety and efficacy. The purpose of this study was to present results and complications following this procedure in a large multicenter series.We reviewed the outcomes of all patients who had been treated with the modified Dunn procedure by five surgeons from separate tertiary-care institutions. All slipped capital femoral epiphyses were defined as unstable according to the Loder criteria. Patients with less than one year of follow-up and those with an underlying endocrinopathy or syndrome were excluded. All surgical procedures were performed by pediatric orthopaedic surgeons who had specific training in the modified Dunn procedure. Operative reports, outpatient records, and follow-up radiographs were used to determine the demographic information, type of fixation, final slip angle, presence of osteonecrosis, and any additional complications. Standardized surveys were administered to determine the pain level (0 to 10 scale), satisfaction (0 to 100 scale), function (modified Harris hip score, 0 to 91 scale), and activity level (UCLA [University of California Los Angeles] activity score, 0 to 10 scale) at time of the most recent follow-up.Twenty-seven patients (twenty-seven hips) with a mean of 22.3 months (range, twelve to forty-eight months) of follow-up met the inclusion criteria. Four patients (15%) had broken implants at three to eighteen weeks after surgery and required revision fixation. Seven patients (26%) developed osteonecrosis at a mean of 21.4 weeks (range, ten to thirty-nine weeks), with each surgeon having at least one case of osteonecrosis. The mean slip angle at the time of the most recent follow-up was 6° (95% confidence interval, 2° to 11°). Patients who did not develop osteonecrosis had significantly better clinical results compared with those who developed osteonecrosis, as demonstrated by a lower mean pain score (0.3 compared with 3.1, p = 0.002), higher level of satisfaction (97.1 compared with 65.8, p = 0.001), higher modified Harris hip score (88.0 compared with 60.0, p = 0.001), and higher UCLA activity score (9.3 compared with 5.9, p = 0.031).This largest reported series of unstable slipped capital femoral epiphyses treated with the modified Dunn procedure demonstrated that the procedure is capable of restoring anatomy and preserving function after a slip but that implant complications and osteonecrosis can and do occur postoperatively.Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.