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Hyponatraemia has been reported to be a potent predictor of poor outcome in patients hospitalized for heart failure (HF). The aim of the study was to determine the prevalence and prognostic significance of hyponatraemia in a large cohort of HF outpatients followed in clinics participating in the Danish Heart Failure Clinics Network.The study population consisted of consecutive patients referred for HF management in 18 Danish heart failure clinics. Overall, 2863 patients (83%) had a normal plasma sodium (p-sodium) level and 602 patients (17%) had hyponatraemia with a p-sodium level <136 mmol/L. Outcome data were obtained from a validated, national registry. Patients were elderly with a mean age of 68 years. The mean P–[Na+] was 139.6 ± 2.4 mmol/L among patients with normonatraemia and 132.4 ± 3.2 mmol/L among patients with hyponatraemia. In multivariate Cox Proportional Hazard Models adjusted for confounders (age, gender, hospitalization within the last 90 days, loop diuretics, creatinine level, systolic blood pressure, New York Heart Association class III–IV, left ventricular ejection fraction <0.46, ischaemic heart disease and diabetes) hyponatraemic patients had increased risk of hospitalization or death [hazard ratio (HR) 1.2 (95% confidence interval (CI) 1.0–1.4, P = 0.011)]. Hyponatraemia was also an independent predictor of all-cause mortality [HR 1.5 (95% CI 1.2–1.9, P< 0.001)]. There was no interaction between hyponatraemia and the covariables on outcome in the multivariable models.The presence of hyponatraemia in outpatients with HF is associated with increased risk of hospitalization or death.