INTRODUCTION: Frailty, a physiological decline in functional capacity, may influence nonunion risk. This study aimed to investigate the association between frailty, as measured by the modified Frailty Index-5 (mFI-5), and the likelihood of nonunion fractures of the upper extremity. METHODS: This retrospective cohort study utilized the national inpatient sample (NIS) from 2015 to 2019. Patients aged 18 and older with upper extremity fractures, identified by ICD-10-CM codes, were included. Patients were categorized into routine healing and nonunion groups. Frailty was assessed using the mFI-5, classifying patients as robust, prefrail, frail, or severely frail. Multivariate logistic regression, controlling for age, sex, and Injury Severity Score (ISS), was performed to determine the association between frailty and nonunion. RESULTS: The study included 21,618 patients, with 3782 presenting with nonunion fractures. The median age was 69 years, and 60.5% were female. The most common fracture types in the routine healing group were forearm (40.1%), clavicle (18.4%), and humerus (16.9%), while in the nonunion group, humerus (30.4%) and scapula (32.1%) were most common. Multivariate logistic regression showed that frail and severely frail patients had a decreased risk of nonunion (OR 0.751 and 0.705, respectively, p <
0.001). Each unit increase in mFI-5 score was associated with a decreased risk of nonunion (OR 0.868, p <
0.001). Sub-analysis revealed a decreased risk of nonunion with increasing frailty for humerus, clavicle, scapula, and phalanx fractures, but no significant association for wrist, forearm, or metacarpal fractures. CONCLUSION: Contrary to expectations, increasing frailty, as measured by the mFI-5, was associated with a decreased risk of nonunion fractures in the upper extremity. This paradoxical finding may be due to closer medical supervision and improved treatment compliance in frail patients. Further prospective studies are needed to explore the complex interplay between frailty, treatment adherence, and fracture healing.