Background. Hypovitaminosis D is linked to unstable health in older adults. Our objectives were to determine (i) the difference in length of stay (LOS) in geriatric acute care unit between inpatients with and without hypovitaminosis D and (ii) whether there was a linear association between serum 25-hydroxyvitamin D (25OHD) concentration and LOS. Methods. 253 inpatients admitted in 2008 to the geriatric acute care unit of Angers University Hospital, France, (mean age±standard deviation, 86.2 ± 6.0 years; 66.8% female) were included in this historical cohort study. LOS was calculated by subtracting day of admission from day of discharge. Hypovitaminosis D was defined as 25OHD≤50 nmol/L at the time of admission. Age, gender, place of life, functional independence, reason for admission, number of acute diseases and comorbidities, use of vitamin D supplements, and creatinine clearance were used as confounders. Results. Participants with hypovitaminosis D had longer LOS than their counterparts (15.2 ± 8.2 days versus 12.1 ± 7.0 days, ), underlining a mean difference of 3 days. 25OHD concentration inversely correlated (, ) and was inversely associated with LOS (adjusted [95%CI: −0.14; −0.02], ). Conclusions. We found an inverse linear association between serum 25OHD concentrations and LOS in a geriatric acute care unit. Participants with 25OHD>50 nmol/L were hospitalized on average 3 days less than those with 25OHD≤50 nmol/L.