Background <p>Phosphate is essential for cellular metabolism, muscle, and brain function. Although electrolyte imbalances are common in older patients, the impact of mild hypophosphatemia remains unclear.</p> Aims <p>We investigate whether serum phosphate level is associated with clinical outcomes in geriatric inpatients.</p> Methods <p>We performed a retrospective cohort study of patients aged ≥ 75 years admitted to the geriatric wards of Lausanne University Hospital between 2019 and 2024. Patients with informed consent and phosphate measurements during hospitalization were included. Outcomes included length of hospital stay, functional and cognitive decline, early (30-day) and one-year rehospitalization rate, and one-year mortality. Mild hypophosphatemia was defined as phosphate &lt; 0.81 mmol/L.</p> Results <p>Amongst 1.287 eligible patients, 190 (15.1%) had mild hypophosphatemia. Mild hypophosphatemia was not associated with mortality or length of stay; however, it was associated with increased risk of rehospitalization and lower cognitive performance. After correction for confounding variables, phosphate was not an independent predictor of these outcomes; age remained the main factor. Hypophosphatemia was more common among patients discharged to higher-care settings.</p> Discussion <p>Mild hypophosphatemia was more prevalent than previously reported, likely reflecting the frailty and multimorbidity of hospitalized geriatric patients. Its associations with rehospitalization, lower cognitive performance, and discharge to higher-care settings suggest an increased vulnerability of mild hypophosphatemia patients rather than a direct causal effect of phosphate on clinical outcomes.</p> Conclusions <p>Mild hypophosphatemia is frequent in hospitalized older adults and is associated with frailty markers but is not an independent prognostic factor of poor health outcomes: it likely reflects underlying vulnerability.</p>