Background: Clinical importance of hyponatremia in ST-elevation acute myocardial infarction (STEMI) in the era of primary intervention has not been fully understood. The aim of this study was to investigate the impact of hyponatremia on outcomes in patients with STEMI and secondarily to investigate the contribution of arginine vasopressin (AVP) to hyponatremia in STEMI. Methods and Results: Hyponatremia was defined as a sodium concentration <136mmol/L at 72h after hospitalization. First, the short-term (in-hospital mortality or congestive heart failure (CHF)) and long-term prognosis (cardiac death, re-admission for CHF) in STEMI patients was conducted. Second, the relationship between serum sodium level and plasma AVP was investigated. In hyponatremic patients the incidence of in-hospital heart failure was significantly greater (P=0.0018), long-term cardiac death was a higher trend (17.2% vs. 6.3%, P=0.19) and re-admission due to CHF was significantly more frequent (20.7% vs. 4.5%, P=0.0024). Plasma AVP level was higher in the hyponatremia group (4.5 vs. 2.7pg/ml, P=0.003), and it had a negative correlation with serum sodium level (r=-0.28, P=0.02). Conclusions: Hyponatremia was frequently found in the early phase of STEMI, and associated with heart failure in both short- and long-term outcomes. Non-osmotic secretion of AVP could be involved in hyponatremia in STEMI patients. (Circ J 2011; 75: 1927-1933)