Kt/V(urea) (Kt/V) depends on the method applied for its evaluation. Our aim was to compare Kt/V obtained using the conductivity online method and that calculated from urea measurements. Studies were carried out in 40 patients. A stable dialysis schedule was maintained during the study. Online Kt/V was measured every week or 4 consecutive months. Single pool Kt/V (spKt/V) was calculated from urea estimations in the fourth week of the first month and in the last week of the fourth month of studies, using the formulas: (1)spKt/V = -ln(Ct/Co), where Ct is the postdialysis urea concentration obtained at the end of dialysis, Co the predialysis urea concentration obtained before the start of the blood pump; (2)spKt/V = -ln(R - 0.008 x t - f x UF/W), where R is the Ct/Co, t the duration of HD session, f=1.0, UF is the ultrafiltration volume (l), W is the body weight after the HD session; and (3)spKt/V + -ln(R - 0.008 x t) + (4 - 3.5 x R) x UF/W. The equilibrated Kt/V (eKt/V) was calculated as (3)spKt/V - {0.47 x [(3)spKt/V]/t} + 0.02. Correlation analysis was performed between all obtained Kt/V. Weekly online Kt/V was stable during 4 months of studies. In the first month, the respective values of online Kt/V, (1)spKt/V, (2)spKt/V, (3)spKt/V, and eKt/V were 1.15+/-0.14, 1.16+/-0.14, 1.38+/-0.17, 1.36+/-0.20, and 1.22+/-0.13. In the fourth month, these values were 1.17+/-0.14, 1.16+/-0.17, 1.38+/-0.22, 1.35+/-0.20, and 1.22+/-0.18. The respective values of Kt/V, estimated in the first and fourth month, were not different and showed a positive correlation: the highest one occurred between online Kt/V estimated at the indicated study periods (r=0.713, p=0.0000). Online Kt/V was significantly lower than (2)spKt/V, (3)spKt/V, and eKt/V. Correlation coefficients between online Kt/V, spKt/V, and urea reduction ratio did not exceed 0.490. Our studies show that Kt/V obtained using online monitoring indicates a lower intermittent hemodialysis adequacy that those calculated from urea measurements. This difference has to be remembered in application of results to clinical practice.