BACKGROUND: Major emergency abdominal surgery (MEAS) has high morbidity and mortality rates. With an aging and frail population, understanding the impact of frailty on postoperative complications is crucial. METHODS: This prospective cohort study was conducted at a major university hospital in Denmark from October 1, 2020, to January 1, 2024. A total of 733 patients undergoing MEAS were assessed for frailty using the clinical frailty scale (CFS) at admission and grouped into low (CFS 1-3), moderate (CFS 4-6), and high (CFS 7-9) frailty categories. Postoperative complications were classified by the Clavien-Dindo score and comprehensive complication index (CCI) until discharge. RESULTS: Patients with CFS one to three had 140 complications per 100 patients, CFS four to six had 267 complications per 100 patients, and CFS seven to nine had 297 complications per 100 patients (p <
0.002). Increased frailty was associated with higher severity of complications (Clavien-Dindo score). Median CCI scores were 8.7 (CFS 1-3), 29.6 (CFS 4-6), and 38.2 (CFS 7-9) (p <
0.002). The cumulative risk of complications was higher in patients with CFS four to six and seven to nine. CONCLUSION: Higher frailty scores correlate with an increased number and severity of complications as well as a greater overall burden of postoperative complications. The clinical frailty scale is effective for preoperative identification of high-risk patients. TRIAL REGISTRATION: The study was approved by the Capital Region of Denmark (P-2020-1166 and R-21038079) and the Danish Data Protection Agency (P-2021-431).