Having an accurate estimation of total body water (TBW) is essential for the evaluation of dialysis efficacy in peritoneal dialysis (PD) patients. In this study, TBW volumes were measured by tritium dilution (TBW(THO)) in 165 PD patients and compared with TBW calculations according to the Watson formulas. An alternative anthropometric formula based on the present PD population was also developed and validated in an independent sample of 29 PD patients. Furthermore, the relation between TBW(THO) and body surface area (BSA) according to the formula of Gehan was analyzed. Body composition was assessed by a four-compartment model, based on measurements of TBW(THO) and total body potassium. Mean values of TBW by the Watson formulas were almost identical to TBW(THO), and the correlation coefficient for the relationship of calculated to measured volumes was 0.89 (P = 0.001). However, both anthropometric formulas-the Watson formulas as well as the new, alternative one-overestimated TBW in obese patients and vice versa in lean patients. Similarly, TBW was underestimated in patients who were overhydrated. The correlation coefficient between TBW(THO) and BSA was 0.708 for males and 0.797 for females (P = 0.0001 for both). In obese patients, the relationship was even closer (r = 0.924 and 0.911, respectively). In conclusion, anthropometric formulas to calculate TBW showed a considerable intraindividual variability compared with measured values. This was related to body composition features, such as degree of obesity and hydration. In contrast, BSA correlated closely to TBW in obese individuals. TBW as estimated by anthropometric formulas must be analyzed with caution, especially in the very obese or very lean patient.