SUMMARY: Hyponatremia has been linked to an increased risk of osteoporosis and fractures. We found an increased hazard ratio of major osteoporotic fractures adjusted for potential confounders, including osteoporosis and medication. A reduced BMD was not sufficiently explaining the association. Our data indicate that hyponatremia should be considered a risk factor for osteoporosis and fractures. INTRODUCTION: Hyponatremia is the most common electrolyte disorder in clinical practice and could be a risk factor for both osteoporosis and fractures. Mild hyponatremia has traditionally been regarded as a benign and asymptomatic condition; however, data from large population and animal studies have led to a reappraisal of this view. The purpose of this study was to evaluate the association of hyponatremia with osteoporosis and major osteoporotic fractures (MOF) in women. METHODS: This is a historical cohort study with fracture follow-up. The study consisted of 5610 patients with available serum sodium and a bone density measurement. Information on potential risk factor was obtained through a questionnaire. Additional information on medication, comorbidities, and fractures was obtained through national registries. RESULTS: Hyponatremia was associated with significant lower T-scores at total hip and a borderline significant lower T-score at femoral neck in the multivariate analysis. No association was found between hyponatremia and the lumbar spine T-score. Hyponatremia was associated with an increased hazard ratio of sustaining a MOF in the period from 6 months prior to 12 months after serum sodium measurement. Finally, data showed a relationship with increasing serum sodium and an increasing T-score estimate and a decreasing hazard ratio of MOF. CONCLUSIONS: Our data suggest that hyponatremia in women increases the risk of osteoporosis and MOF. The increased risk of MOF was independent of osteoporosis.