RATIONALE: Although associated with adverse outcomes in other cardiopulmonary conditions, the prognostic value of hyponatremia, a marker of neurohormonal activation, in patients with acute pulmonary embolism (PE) is unknown. OBJECTIVES: To examine the associations between hyponatremia and mortality and hospital readmission rates for patients hospitalized with PE. METHODS: We evaluated 13,728 patient discharges with a primary diagnosis of PE from 185 hospitals in Pennsylvania (January 2000 to November 2002). We used random-intercept logistic regression to assess the independent association between serum sodium levels at the time of presentation and mortality and hospital readmission within 30 days, adjusting for patient (race, insurance, severity of illness, use of thrombolytic therapy) and hospital factors (region, size, teaching status). MEASUREMENTS AND MAIN RESULTS: Hyponatremia (sodium ≤135 mmol/L) was present in 2,907 patients (21.1%). Patients with a sodium level greater than 135, 130-135, and less than 130 mmol/L had a cumulative 30-day mortality of 8.0, 13.6, and 28.5% (P < 0.001), and a readmission rate of 11.8, 15.6, and 19.3% (P < 0.001), respectively. Compared with patients with a sodium greater than 135 mmol/L, the adjusted odds of dying were significantly greater for patients with a sodium 130-135 mmol/L (odds ratio [OR], 1.53; 95% confidence interval [CI], 1.33-1.76) and a sodium less than 130 mmol/L (OR, 3.26; 95% CI, 2.48-4.29). The adjusted odds of readmission were also increased for patients with a sodium of 130-135 mmol/L (OR, 1.28; 95% CI, 1.12-1.46) and a sodium less than 130 mmol/L (OR, 1.44; 95% CI, 1.02-2.02). CONCLUSIONS: Hyponatremia is common in patients presenting with PE, and is an independent predictor of short-term mortality and hospital readmission.