We tested a new biosensor for urea monitoring in the ultrafiltrate during PFD in a group of 5 hemodialyzed stable patients. The inspection of the UF-urea profile reflects the dynamical changes of the plasma urea concentration during diffusive dialysis and allows the fitting of the main mathematical models of urea kinetics. The biosensor efficiency was 98.4% on average (SD: 1.5%) at Uf fluxes varying from 45 to 55 ml/min (mean: 51 ml/min; SD: 3.2) and at Uf-urea concentrations varying from 23 to 165 mg/dl. The mean difference between Uf-urea determined by the laboratory method and Uf-urea assayed by the biosensor was -1.07 mg/dl and the 95% confidence interval ranged from -2.01 to 0.13 mg/dl. The mean difference between laboratory plasma urea and Uf-urea from the biosensor was on average -1.9 mg/dl and the estimated limits of agreement with a confidence of 95% were -3.16 and 0.64 mg/dl. Comparison between kinetic models and experimental profiles of plasma urea decrease, evaluations of recirculation and post-dialytic rebound, the role of Kt/V on-line during dialysis were the preliminary clinical applications of this biosensor.