Background: Although reducing 30-day hospital readmissions is now a priority in neurosurgical quality improvement, postoperative emergency room (ER) visits have remained poorly understood, in particular, in populations with limited access to outpatient care. After endoscopic transsphenoidal surgery, the unique pathologic entities treated can engender a variety of surgical and metabolic complications-often dangerous, delayed, and nonspecific in presentation. We sought to characterize the causes and timing of ER visits-with or without readmission-in a socioeconomically disadvantaged population., Methods: We reviewed all the patients undergoing ETS by our skull base team from 2009 to 2017 to determine their socioeconomic profile and causes of ER visits. For external validation, we compared our results with national data from the American College of Surgeons National Surgical Quality Improvement Program., Results: Of the 229 patients, 35 (15.3%) had visited the ER within 30 days of discharge. Of these 229 patients, 21 (9.2%) were readmitted, and 109 (47.6%) were insured by Medicaid. The most common reasons for the ER visits were headache (8 of 35 [22.9%]), hyponatremia (7 of 35 [20.0%]), and epistaxis (5 of 35 [14.3%]). The most common cause for readmission was symptomatic hyponatremia (6 of 21 [28.6%]). The other reasons for readmission included meningitis (2 of 21 [9.5%]), adrenal crisis (2 of 21 [9.5%]), and cerebrospinal fluid leakage (1 of 21 [4.8%]). The average time to readmission was 10.9 days. According to the National Surgical Quality Improvement Program database, the 30-day readmission rate was 5.5%, of which 22.2% were for hyponatremia., Conclusion: Delayed hyponatremia accounts for a large proportion of ER visits and unplanned readmissions. Close follow-up with diligent, multidisciplinary care might reduce the number of ER visits and readmission in this population., (Copyright © 2018 Elsevier Inc. All rights reserved.)