Urea kinetic modeling (UKM) was carried out to evaluate the water control in patients undergoing long-term hemodialysis. In 21 patients on chronic dialysis, the results of two different determinations of the total body water, i.e. by the deuterium oxide (D2O) method and by UKM, were compared. A correlation was observed with Y = 0.98X + 5.9 (P less than 0.01, r = 0.91) for males and Y = 0.97X + 6.2 (P less than 0.01, r = 0.90) for females, indicating that UKM is useful for determining the total body water. In addition, the 21 patients were classified into two groups based on their ECG findings, and UKM was performed in the two groups for comparison. The values of the total body water were 68.5 +/- 4.0% and 68.6 +/- 6.2% of body weight, respectively, with no significant difference between them. However, the CTR was 50.2 +/- 2.8% and 46.2 +/- 4.4%, respectively. Thus, not only X-ray examinations but also UKM should be carried out to determine the standard weight (SW-DW) in the presence of cardiac dysfunction. Instructions for water control were given when the total body water exceeded 73% during the UKM measurements over 5 years, which allowed an optimal total body water to be maintained thereafter. The possible influence of erythropoietin (EPO) was also examined by performing UKM in 8 cases receiving EPO and 7 cases without EPO. No noticeable difference was observed between the two groups. 17 patients using a polyacrylonitrile (PAN) membrane were further divided into those with and those without EPO to evaluate the possible influence of the PAN membrane-EPO combination. As a result, no significant difference was demonstrated, suggesting that UKM can be satisfactorily performed under such conditions. In 11 patients on chronic dialysis, the levels of alpha-human atrial natriuretic peptide (alpha-hANP) were measured pre- and post-dialysis to examine the relationship between the change in alpha-hANP and rate of weight loss (%). A significant correlation was observed with Y = 12.8X + 5.1 (P less than 0.05, r = 0.68). In conclusion, UKM was found to be useful for evaluating the water control and for assessing the optimal dialysis in patients receiving long-term hemodialysis.