BACKGROUND CONTEXT With the increasing frequency of spinal fusion surgery, there has been increased pressure on providers and hospital systems to decrease cost and increase the value of this procedure. One potential area of improvement is decreasing length of hospital stay. Minimally invasive surgery (MIS) of the spine is designed to improve outcomes by minimizing approach. Previous studies looking at length of stay after MIS spine surgery are limited by their retrospective nature, single-intuition, small sample size, poor long-term follow-up, as well as their ability to collect outcomes. Identification of pre-operative factors that can predict shorter length of stay would allow surgeons to stratify what surgeries can be performed in an outpatient setting or in an ambulatory surgery center as a means of further cost containment. PURPOSE The goal of this study was to identify pre-operative factors that would be predictive of shorter length of stay (LOS) after minimally invasive transforaminal interbody fusion (MIS TLIF) or lateral lumbar interbody fusion (LLIF). STUDY DESIGN/SETTING The MSSIC is a statewide, prospective, longitudinal, multicenter quality improvement registry. We retrospectively analyzed a subset of MSSIC patients who had undergone TLIF or LLIF from a single surgeon's experience. PATIENT SAMPLE Patients that underwent one or two level MIS TLIF or LLIF from 2015 to 2016 were eligible for enrollment into this study. Inclusion criteria include: spondylosis; intervertebral disc disorders; other and unspecified disorders of the back; other acquired musculoskeletal deformity (Grade 1 or 2 spondylolisthesis); other congenital musculoskeletal anomalies (Grade 1 or 2 spondylolisthesis); and complications peculiar to certain specified procedures. Exclusion criteria include: patient age is younger than 18 years; patient is incarcerated; patient's medical records are unavailable or inaccessible; and cases of moderate (25°–50°) and severe (>50°) scoliosis. Additional exclusion criteria include: pure thoracic cases; tumor; meningitis; preexisting spinal infection related to surgery; other disorders of bone and cartilage, spinal deformity, Grades 3 and 4 spondylolistheses; congenital anomalies of the nervous system; traumatic fracture; and spinal cord injury. OUTCOME MEASURES The primary outcome measure for this study was LOS. A LOS ≥4days was considered prolonged. METHODS Variables recorded prospectively include medical history, indications for surgery, surgical details, postoperative hospital course and any comorbidities. The modified frailty index was calculated for each patient; briefly, it is a numerical score that is dependent on the patient's past medical history. In addition, daily oral morphine equivalents (OME) were also calculated preoperatively (in milligrams) and considered for analysis. Multivariable logistic regression models were constructed to identify potential risk factors for increased LOS after MIS TLIF or LLIF. We assigned length of stay ≥4days as prolonged. RESULTS We identified 73 patients who met the inclusion criteria. Of these patients, 80.8% underwent a 1 level procedure, 19.2% underwent a 2 level procedure, and 13.6% experienced prolonged discharge. Preoperative OME did not affect length of stay (17.9 ±2.1 mg vs. 18.4±5.6 mg, p=.998). Number of levels of fusion did not affect length of stay. Modified frailty index was significantly higher in patients with delayed discharge (2.6±0.4vs. 1.6±0.15, p=.021). Following a multivariable regression, every 1-point on a cormorbidity scale increased the odds of belated discharge by 79% (p=.048). No affect was observed with morphine milligram equivalents. CONCLUSIONS Modified frailty index predicts length of stay after MIS TLIF and LLIF. Preoperative opioid use does not affect length of stay.