Alterations in the plasma water sodium concentration ([Na⁺]_pw) result from changes in the total exchangeable sodium (Na_e), total exchangeable potassium (K_e), and total body water (TBW). The empirical relationship between the [Na⁺]_pw and Na_e, K_e, and TBW was originally demonstrated by Edelman et al (1) where: [Na⁺]_pw = 1.11(Na_e + K_e)/TBW - 25.6 (Eq. 1). Based on Eq. 1, alterations in the [Na⁺]_pw can be predicted by considering changes in the mass balance of Na⁺, K⁺ and H₂O. In accounting for the mass balance of Na⁺, K⁺, and H₂O in patients on peritoneal dialysis, considerations must also be taken to determine the modulating effect of dialysate clearance of Na⁺ and K⁺ and fluid changes resulting from this therapeutic modality on the [Na⁺]_pw. In this article, we derive a new formula for predicting alterations in the plasma Na⁺ concentration ([Na⁺]_p) in patients on peritoneal dialysis taking into consideration the empirical relationship between the [Na⁺]_pw and Na_e, K_e, and TBW (Eq. 1) as well as changes in mass balance of Na⁺, K⁺, and H₂O.