BACKGROUND : The modified 5-item frailty index (mFI-5) is a concise comorbidity-based risk stratification tool that has been shown to predict the occurrence of adverse outcomes following various orthopedic surgeries. METHODS : The 2012-2016 American College of Surgeons - National Surgical Quality Improvement Program (ACS-NSQIP) dataset was used to identify patients undergoing an elective 1- to 2-level posterior lumbar fusion for degenerative lumbar pathology. The mFI-5 score was calculated based on the presence of the 5 co-morbidities: congestive heart failure within 30 days prior to surgery, insulin-dependent or noninsulin-dependent diabetes mellitus, chronic obstructive pulmonary disease or pneumonia, partially dependent or totally dependent functional health status at time of surgery, and hypertension requiring medication. Multivariate analysis was used to assess the independent impact of increasing mFI-5 score on postoperative morbidity while controlling for baseline clinical characteristics. RESULTS : Increasing mFI-5 score versus mFI-5 = 0 was associated with higher odds of any complication (mFI-5 ≥2: odds ratio [OR] 1.45; mFI-5 = 1: OR 1.22), 30-day readmissions (mFI-5 ≥2: OR 1.46; mFI-5 = 1: OR 1.18), and nonhome discharge (mFI-5 ≥2: OR 1.80; mFI-5 = 1: OR 1.16). Higher mFI-5 score was significantly associated with increased risks of superficial surgical site infection, deep surgical site infection, unplanned reoperation, any medical complication, pneumonia, unplanned intubation, postoperative ventilator use, progressive renal insufficiency, acute renal failure, urinary tract infection, stroke, myocardial infarction, bleeding requiring transfusion, sepsis, and septic shock. CONCLUSIONS : Higher mFI-5 scores were associated with increased postoperative morbidity following elective 1- to 2-level posterior lumbar fusions.