Dehydration is an overlooked modifiable risk factor that should be optimized prior to elective total hip arthroplasty (THA) to reduce postoperative complications and inpatient costs. All primary THA from 2005 - 2019 were queried from the National Surgical Quality Improvement Program database, and patients were compared based on dehydration status: blood urea nitrogen (BUN): creatinine ratio (Cr) (BUN/Cr) < 20 (nondehydrated), 20 ≤ BUN/Cr ≤ 25 (moderately dehydrated), 25 < BUN/Cr (severely dehydrated). A subgroup analysis involving only elderly patients > 65 years and normalized gender-adjusted Cr values was also performed. The analysis included 212,452 patients who underwent THA. Adjusted multivariate logistic regression analysis showed that the severely dehydrated cohort had a greater risk of overall complications, postoperative anemia requiring transfusion, nonhome discharge, and increased length of stay (all p < 0.01). Among the elderly, dehydrated patients had a greater risk of postoperative transfusion, cardiac complications, and nonhome discharge (all p < 0.01). BUN/Cr > 20 is an important preoperative diagnostic tool to identify at-risk dehydrated patients. Providers should optimize dehydration to prevent complications, decrease costs, and improve discharge planning. (Journal of Surgical Orthopaedic Advances 33(1):017-025, 2024).