The objectives of this study were: (1) to compare removal of the middle molecular (MM) and small uremic retention solutes; (2) to investigate if MM removal can be assessed by UV-absorbance at the wavelength of 297 nm during various dialysis treatment modalities. Seven uremic patients, four females and three males, mean age 58.1 +/- 8.7 years, were included into the study during 28 chronic hemodialysis sessions. A parameter, reduction ratio (RR) in percentage, was calculated for a small uremic retention solute urea, for a MM retention solute beta2-microglobulin (B2M), and for UV-absorbance at the wavelength of 297 during different dialysis modalities: conventional hemodialysis (HD), high flux hemodialysis (HF-HD), and postdilutional online hemodiafiltration (HDF) with different parameter settings. Achieved results were compared regarding mean values and SD, and by systematic and standard errors (BIAS +/- SE). It was found that RR is similar for small and MM uremic retention solutes in case of dialysis modality with the highest convective transport, HDF (78.9 +/- 8.1% for urea and 78.1 +/- 6.8% for B2M, N=7). Moreover, RR of small uremic retention solutes can be estimated with sufficient accuracy by UV-absorbance at 297 nm in the spent dialysate for all modalities (BIAS +/- SE: 1.7 +/- 4.0%, N=28), and for MM uremic retention solutes only for HDF (BIAS +/- SE: 1.1 +/- 7.1%, N=7). The results should be confirmed by appropriate kinetic modeling in the next studies.