Background: Approximately 1 billion people worldwide have Vitamin D deficiency. The aim of this study was to compare Vitamin D status and serum 25-hydroxyvitamin D (25(OH)D) concentrations among adults sampled in the community, in outpatient clinics, as hospital inpatients and in nursing homes in the West of Ireland. The secondary aim was to determine the associations between length of hospital stay (inpatients) at the time of serum 25(OH)D sampling and Vitamin D status.Methods: A cross-sectional study was carried out. Patients who had serum 25(OH)D analysis carried out in Galway University Hospitals (January 2011-December 2015) were identified following interrogation of the electronic laboratory data system. Baseline demographics, location, and date of sample collection were recorded. Vitamin D deficiency was defined as a serum 25(OH)D concentration <25 nmol/L.Results: In total, 24,302 patient samples were eligible for inclusion: community 15,319; outpatient clinics 6,371; inpatients 2,339; and nursing home residents 273. Vitamin D deficiency was more common in nursing home residents than inpatients, or those sampled in outpatient clinics or in the community (42% vs 37% vs 17% vs 13%; p < .001). Inpatients sampled further into their hospital stay (>= 3 days) had greater Vitamin D deficiency than inpatients sampled on 0-2 days (p = .007). Season (p < .001), sex (p < .001), and age (p < .001) were associated with 25(OH)D concentrations. Vitamin D deficiency was more common in Winter/Spring, in males, and in those aged >= 80 years.Conclusions: Nursing home residents and inpatients are at the highest risk for Vitamin D deficiency. Season, sex, age, and day of hospital stay on which serum 25(OH)D concentrations were sampled were associated with Vitamin D status.