OBJECTIVES: To analyze patients, injury pattern, and treatment of femoral neck fractures in young patients with femoral neck fractures associated with shaft fractures (assocFNFs) to improve clinical outcomes. The secondary goal was to compare this injury pattern to that of young patients with isolated femoral neck fractures (isolFNFs). METHODS: Design: Retrospective multicenter cohort series Setting: 26 North American Level-1 trauma centers Level of Evidence: Level III Patient Selection Criteria: Skeletally mature patients, <50 years old, treated with operative fixation of a femoral neck fracture with or without an associated femoral shaft fracture. Outcome Measures and Comparisons: The main outcome measurement was treatment failure defined as nonunion, malunion, avascular necrosis, or subsequent major revision surgery. Odds ratios for these modes of treatment were also calculated. RESULTS: Eighty assocFNFs and 412 isolFNFs evaluated in the study were different in terms of patients, injury patterns, and treatment strategy. AssocFNF patients were younger (33.3±8.6 vs. 37.5±8.7 years old, p<0.001), greater in mean BMI (29.7 vs. 26.6, p<0.001), and more frequently displaced (95% vs. 73%, p<0.001), “vertically oriented” Pauwels’ type 3, p<0.001 (84% vs. 43%) than for isolFNFs, with all p values <0.001. AssocFNFs were more commonly repaired with an open reduction (74% vs. 46%, p<0.001) and fixed angle implants (59% vs. 39%) (p<0.001). Importantly, treatment failures were less common for assocFNFs compared to isolFNFs (20% vs. 49%, p<0.001) with lower rates of failed fixation/ nonunion and malunion (p<0.001 and p=0.002, respectively). Odds of treatment failure (OR=0.270, 95% CI=0.15-0.48, p<0.001), nonunion (OR=0.240, 95% CI=0.10-0.57, p<0.001), and malunion (OR=0.920, 95% CI=0.01-0.68, p=0.002) were also lower for assocFNFs. Excellent or good reduction was achieved in 84.2% of assocFNFs reductions and 77.1% in isolFNFs (p=0.052). AssocFNFs treated with fixed angled devices performed very well, with only 13.0% failing treatment compared to 51.9% in isolFNFs treated with a fixed angle constructs (p=<0.001) and 33.3% in assocFNFs treated with multiple cannulated screws (p=0.034). The study also identified the so-called “shelf sign”, a transverse ≥6mm medial-caudal segment of the neck fracture (forming an acute angle with the vertical fracture line) in 54% of assocFNFs and only 9% of isolFNFs (p<0.001). AssocFNFs with a shelf sign failed in only 5 of 41 (12%) of cases. CONCLUSIONS: AssocFNFs in young patients are characterized by different patient factors, injury patterns, and treatments, than for isolFNFs, and have a relatively better prognosis despite the need for confounding treatment for the associated femoral shaft injury. Treatment failures among assocFNFs repaired with a fixed angled device occurred at lower rate compared to isolFNFs treated with any construct type, and assocFNFs treated with multiple cannulated screws. The radiographic “shelf sign” was found as positive prognostic sign in more than half of assocFNFs and predicting a high rate of successful treatment. LEVEL OF EVIDENCE: Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.