OBJECTIVES: The purpose of this study was to determine whether the SPRINT definition of a "critical-sized defect" (fracture gap at least 1 cm in length and involving over 50% of the cortical diameter) was accurate, to discern which factors predict reoperation in patients with these defects, and to compare the patient-based outcomes of these patients with patients without a critical defect. DESIGN: Therapeutic Cohort Study. SETTING: Level 1 and level 2 trauma centers. PATIENTS: Thirty-seven patients in the SPRINT trial with a critical-sized defect participated. We evaluated these patients for planned and unplanned secondary intervention to gain union. Additionally, we evaluated which other factors predicted the need for reoperation. Finally, the 37 patients with a critical defect were compared with the larger cohort of patients without a defect with respect to demographics, mechanism of injury, fracture characteristics, and patient-based outcome. INTERVENTION: Revision surgery for tibial nonunion. RESULTS: Of the 37 patients with a large fracture gap, 7 patients had a planned secondary procedure. Of the remaining 30 patients in whom the attending surgeon adopted a "watch and wait" strategy, 14 patients (47%) never required additional surgery to gain union. Additional surgery to gain union was less likely in patients treated with a reamed nail (P = 0.04) and in female patients (P = 0.04). Patients with a critical-sized defect were more likely to have a high-energy mechanism of injury (P = 0.001), AO-OTA fracture type 42 B or C (P < 0.001), and location involving the middle third of the tibia (P = 0.02). The 12-month SF-36 physical component summary score in patients with a critical-sized defect was 38.2 ± 10.5 (mean ± SD) compared with 43.3 ± 10.7 in those without a critical defect (P = 0.02, difference = 5.2, 95% confidence interval = 0.8-9.6). CONCLUSIONS: Tibial diaphyseal defects of >1 cm and >50% cortical circumference healed without additional surgery in 47% of cases. This definition of a critical-sized defect is not "critical." However, as compared with the overall cohort of tibial fractures, patients with these bone defects had a higher rate of reoperation and worse patient-based outcomes. Further investigation is required to determine which factors predict union in this challenging fracture to avoid unnecessary secondary surgery. LEVEL OF EVIDENCE: Prognostic level I. See instructions for authors for a complete description of levels of evidence.