With the increasing medical complexity of patients undergoing posterior lumbar surgery, more patients are pharmacologically immunosuppressed to manage chronic conditions. The effects of immunosuppression have become of greater interest across multiple surgical specialties. The goal of the current study was to investigate whether long-term corticosteroid use is independently associated with perioperative adverse outcomes among patients undergoing posterior lumbar surgery. Patients who underwent elective posterior lumbar spine surgery (decompression and/or fusion) were identified in the 2005-2016 National Surgical Quality Improvement Program (NSQIP) database. Patient factors, surgical factors, and 30-day perioperative outcomes for patients taking long-term corticosteroids were compared with those for patients who were not taking these drugs. Propensity matching and multivariate analysis were used to evaluate comparable patients while controlling for potentially confounding variables. In total, 140,519 patients undergoing posterior lumbar spine surgery were identified. Of these, 5243 (3.73%) were taking corticosteroids. After propensity matching and controlling for age, sex, body mass index, functional status, American Society of Anesthesiologists class, and surgical procedure, those taking corticosteroids were at greater risk for any adverse event (odds ratio, 1.45), a serious adverse event (odds ratio, 1.57), a minor adverse event (odds ratio, 1.47), infection (odds ratio, 1.48), reoperation (odds ratio, 1.48), and readmission (odds ratio, 1.47) (P≤.001 for each). The findings confirmed that long-term corticosteroid use is associated with significant increases in perioperative adverse outcomes for patients undergoing elective posterior lumbar surgery, even with matching and controlling for potentially confounding variables. These findings can guide patient counseling and preemptive interventions before surgery for this patient population. [Orthopedics. 2021;44(3):172-179.].