Low serum sodium concentration is an independent predictor of mortality in patients with cirrhosis, but its prevalence and clinical significance is unclear. To evaluate prospectively the prevalence of low serum sodium concentration and the association between serum sodium levels and severity of ascites and complications of cirrhosis, prospective data were collected on 997 consecutive patients from 28 centers in Europe, North and South America, and Asia for a period of 28 days. The prevalence of low serum sodium concentration as defined by a serum sodium concentration < or =135 mmol/L, < or =130 mmol/L, < or =125 mmol/L, and < or =120 mmol/L was 49.4%, 21.6%, 5.7%, and 1.2%, respectively. The prevalence of low serum sodium levels (<135 mmol/L) was high in both inpatients and outpatients (57% and 40%, respectively). The existence of serum sodium <135 mmol/L was associated with severe ascites, as indicated by high prevalence of refractory ascites, large fluid accumulation rate, frequent use of large-volume paracentesis, and impaired renal function, compared with normal serum sodium levels. Moreover, low serum sodium levels were also associated with greater frequency of hepatic encephalopathy, spontaneous bacterial peritonitis, and hepatorenal syndrome, but not gastrointestinal bleeding. Patients with serum sodium <130 mmol/L had the greatest frequency of these complications, but the frequency was also increased in patients with mild reduction in serum sodium levels (131-135 mmol/L). In conclusion, low serum sodium levels in cirrhosis are associated with severe ascites and high frequency of hepatic encephalopathy, spontaneous bacterial peritonitis, and hepatorenal syndrome.