BACKGROUND: The purpose of the present multicenter cohort study was to compare the rates of reoperation in elderly patients with pertrochanteric and subtrochanteric fractures that had been treated with a short or long intramedullary nail. METHODS: Eleven orthopaedic departments across Denmark delivered data that had been obtained from the Danish Multidisciplinary Hip Fracture Registry (DMHFR) and a review of health-care records. With use of surgical procedure codes for pertrochanteric and subtrochanteric fractures that had been treated with an intramedullary nail, the DMRHF was searched to identify patients who had been ≥65 years of age between 2008 and 2013. The health-care records for patients who had had major reoperations, defined as any reoperation with the exclusion of simple hardware removal, were retrieved from the DMHFR, were reviewed for the type of intramedullary nail and missed reoperations, including peri-implant fractures, within 2 years of follow-up. Crude logistic regression analysis with adjustment for age, sex, and comorbidity was performed, yielding odds ratios (ORs) with 95% confidence intervals (CIs). RESULTS: Of 2,245 pertrochanteric fractures, 1,867 were treated with a short intramedullary nail and 378 were treated with a long intramedullary nail. The rate of major reoperations was 4.0% in the short intramedullary nail group and 6.3% in the long intramedullary nail group. This yielded a crude OR of 1.61 (1.01 to 2.60) and an adjusted OR of 1.67 (1.04 to 2.70). Of 909 subtrochanteric fractures, 308 were treated with a short intramedullary nail and 601 were treated with a long intramedullary nail. The rate of major reoperations was 8.4% in the short intramedullary nail group and 4.0% in the long intramedullary nail group, yielding a crude OR of 0.45 (0.25 to 0.80) and an adjusted OR of 0.45 (0.25 to 0.81). CONCLUSIONS: This is the first study confirming the theory that, for subtrochanteric fractures, a long intramedullary nail has a lower rate of major reoperations compared with a short intramedullary nail. In contrast, a short intramedullary nail has a lower rate of major reoperations compared with a long intramedullary nail for pertrochanteric fractures but with a lower absolute risk reduction compared with the results for subtrochanteric intramedullary nailing. There is a need for validation of these results in other large studies, especially for pertrochanteric fractures. LEVEL OF EVIDENCE: Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.