Background and purpose - Displaced femoral neck fractures (FNFs) are associated with high rates of mortality during the first postoperative year. The Sernbo score (based on age, habitat, mobility, and mental state) can be used to stratify patients into groups with different 1-year mortality. We assessed this predictive ability in patients with a displaced FNF treated with a hemiarthroplasty or a total hip arthroplasty. Patients and methods - 292 patients (median age 83 (65-99) years, 68% female) with a displaced FNF were included in this prospective cohort study. To predict 1-year mortality, we used a multivariate logistic regression analysis including comorbidities and perioperative management. A receiver operating characteristic (ROC) analysis was used to evaluate the predictive ability of the Sernbo score, which was subsequently divided in a new manner into a low, intermediate, or high risk of death during the first year. Results - At 1-year follow-up, the overall mortality rate was 24%, and in Sernbo's low-, intermediate-, and high-risk groups it was 5%, 22%, and 51%, respectively. The Sernbo score was the only statistically significant predictor of 1-year mortality: odds ratio for the intermediate-risk group was 4.2 (95% Cl: 1.5-12) and for the high-risk group it was 15 (95% CI: 5-40). The ROC analysis showed a fair predictive ability of the Sernbo score, with an area under the curve (AUC) of 0.79 (95% CI: 0.73-0.83). Using a cutoff of less than 11 points on the score gave a sensitivity of 61% and a specificity of 83%. Interpretation - The Sernbo score identifies patients who are at high risk of dying in the first postoperative year. This scoring system could be used to better tailor perioperative care and treatment in patients with displaced FNF.