Upper extremity (UE) replantation and revascularization are challenging surgical procedures, with survival rates being 50% to 90%. Preoperative risk stratification is challenging yet crucial as patients with comorbid conditions face increased complications. This study assesses the predictive value of the modified 5-item frailty index (5-mFI) for postoperative complications in these procedures. A retrospective study was done using the ACS NSQIP database (2011-2021) for UE replantation/revascularization. The 5-mFI score assigned points for comorbidities including pulmonary disease, heart failure, diabetes, hypertension, and functional status (0-5 scale; 1 point to each). 5-mFI scores stratified patients into <2 (low-risk) or ≥2 (high-risk) categories. Of 2,305 patients, the mean age and BMI were 53.13 years and 28.53 kg/m2 respectively. The 5-mFI≥2 cohort experienced higher rates of all-cause complications, systemic complications, unplanned readmissions, and return to the operating room on univariate analysis. There was no significant difference in wound complication rates. Multivariable logistic regression showed that a higher 5-mFI was significantly associated with increased risks of all-cause mild and severe systemic complications. Wound complications, length of stay over 30 days, unplanned readmission, and return to the operating room were higher in high-risk patients, however not statistically significant. 5-mFI is an effective tool for evaluating risk in UE replantation and revascularization, correlating high scores with significantly increased postoperative complications. High-risk patients with 5-mFI≥2 also had more reoperations and readmissions. The use of 5-mFI in preoperative assessments can help personalize management, enhancing patient selection and care quality in these complex reconstructions.