Abstract Disclosure: X. Sun: None. B. Leder: None. T.V. Ly: None. E. Franco-Garcia: None. M.B. Bolster: Grant Recipient; Self; Rheumatology Research Foundation Clinician Scholar Educator Award; Genentech, Cumberland, Inc, Corbus, Mitsubishi. Other; Self; Honoraria: The Merck Manual, American Board of Internal Medicine, Practice Update. W. Fan: None. Context: A Fracture Liaison Service (FLS) provides an important mechanism to address vitamin D deficiency (VDD) which is commonly found in patients hospitalized with a fragility fracture. Objectives: To study the prevalence of VDD and its associated clinical outcomes among patients hospitalized for osteoporotic fracture. To explore inpatient strategies of vitamin D repletion in the immediate post fracture setting. Design, Setting and Patients: An observational study of patients admitted to the Massachusetts General Hospital (MGH) with fractures between January 2016 to October 2023 who were cared by the FLS. Exposures and Intervention: A vitamin D repletion regimen consisting of ergocalciferol 50,000 international units (IU) oral daily for a number of days followed by maintenance cholecalciferol of 1000-1500 IU daily was studied in a subset of patients with VDD. Main Outcomes and Measures: Prevalence of VDD among MGH FLS patients. Efficacy of inpatient daily ergocalciferol administration to raise serum 25OHD. Results: Of the 2,951 consecutive patients, 724 (24.53%) were vitamin D deficient as defined by serum 25OHD level of ≤19ng/ml. Men (252 of 897, or 28.09%) were more likely than women (472 of 2,054, or 22.98%) to have VDD (p = 0.003). VDD was seen 41.79% (117 of 280), 24.41% (332 of 1,360) and 20.98% (275 out 1,311) of patients of ≤59, 60-79, and ≥80 years old, respectively (p < 0.00001).Of the 1,303 patients with hip fragility fractures, 327 (25.09%) had VDD. Patients with VDD, despite being younger (78.55±11.75 vs. 80.28±10.47, p=0.018), had longer average length-of-stay (aLOS) than those without VDD did (8.37±7.35 vs. 7.23±4.78 days, p=0.009). Re-admission and non-union rates up to two years following the index hip fracture were not different between the two groups.Among 32 patients, without known malabsorption or liver cirrhosis, who had 25OHD levels available before and after ergocalciferol treatment, each dose of ergocalciferol, on average, raised serum 25OHD by 3.62±2.35 (m±sd) ng/ml. Daily ergocalciferol administration did not change serum calcium and creatinine levels.Patients with chronic liver disease (CLD), who had higher prevalence of VDD (64 of 156 patients, or 41.04%), showed impaired per-dose-of-ergocalciferol 25OHD response (0.87±0.92. n=10) ng/ml. Conclusions and Relevance: Nearly 25% of patients hospitalized for osteoporotic fractures, cared for by MGH FLS, had VDD. Male sex, younger age, and CLD were associated with a higher prevalence of VDD. VDD was associated with, on average, one extra day of hospital stay among patients admitted for hip fracture. Daily ergocalciferol, with each dose of 50,000 IU raising serum 25OHD level by 3-4 ng/ml, followed by maintenance dose of cholecalciferol of 1000 to 1500 IU is potentially a practical way to quickly replete vitamin D among patients with VDD hospitalized for fracture. Presentation: 6/3/2024