Background: There are few reports on the incidence, etiology and mortality of peritoneal dialysis (PD) patients with hyponatremia. Patients and Methods: We identified all the adults who received. Hyponatremia was defined as a serum sodium concentration <135 mmol/L. We enrolled 99 patients who did not show hyponatremic episodes and 297 who showed hyponatremia during follow-up. For evaluating the volume status, all the patients underwent bioelectrical impedance analysis. Results: The incidence of hyponatremia increased as the grade of the Davies risk index increased. The most common cause of hyponatremia was a sodium chloride deficit (39.5%). On the univariate analysis, old age, the presence of hyponatremia during follow-up, hypoalbuminemia, a low residual renal function (RRF) and a high comorbid condition were associated with mortality. On the multivariate analysis, old age, hypoalbuminemia, low RRF and a high comorbid condition were associated with mortality. Conclusion: Accurate evaluation of TBW is important for verifying the etiology of hyponatremia and for treating it. The risk for hyponatremia was associated with higher comorbidity in the PD patients. For the prevention or management of hyponatremia in the PD patients, we would pay more attention to correct any fluid-electrolyte abnormalities and also the underlying comorbidity.