In a comprehensive evaluation of dialysis adequacy, major attention has been recently paid to fluid and Na balance. Removal of Na has been reported to be significantly poorer with automated peritoneal dialysis (APD) than with continuous ambulatory peritoneal dialysis. Only limited data on Na removal with tidal APD have been published. We analyzed peritoneal Na mass balance in 122 separate nightly tidal APD sessions performed by 7 peritonitis-free, clinically stable, patients with negligible residual renal function (< 100 mL urine daily). Correlations with other efficiency measures [ultrafiltration (UF) and small-solute clearances], prescriptive parameters [duration of treatment, initial intraperitoneal fill volume (IPV) and its tidal percentage, and dialysate flux] and peritoneal transport status were tested in univariate and multivariate linear regression models. Removal of Na was 89 +/- 55 mmol per treatment, which correlated with UF (r = 0.29, p = 0.001) and was higher in patients with high-average transport (118 +/- 41 mmol vs. 81 +/- 56 mmol in low-average transporters, p = 0.0004), in whom a significant positive correlation was found with initial IPV and duration of treatment (r = 0.55; 95% confidence interval: 0.21 to 0.77; p = 0.0029; and r = 0.66; 95% confidence interval: 0.38 to 0.83; p = 0.0002 respectively). Removal of Na correlated weakly with UF in tidal APD and showed wide inter-patient variability. It should therefore be measured rather than roughly estimated from UF. Its magnitude exposes the anuric patient on nightly APD with a "dry" day to the risk of Na retention, unless controlled Na intake or dialytic strategies aimed at enhancing Na removal, or both, are implemented.