Convective mass transfer in hemodialysis is associated with ultrafiltration (UF). In the absence of diffusion as in hemofiltration, the convective clearance is equal to S.QF where S is the apparent solute sieving coefficient and QF the UF flow rate, but the convective contribution significantly decreases when diffusion is present. A rigorous calculation of the combined diffusion-convection mass transfer for partially rejected solutes is very complex. In this paper we review various models of mass fluxes found in the literature. Since all these models express the mass flux through the membrane as a linear function of blood and dialysate concentrations with different coefficients, we present a general expression for the hemodiafiltration clearance combining diffusion and convection which can be adapted to each model of mass flux. A surprising result is that the convective contribution to the clearance is, in the limit of dominant ultrafiltration, independent of the solute sieving coefficient, in contrast to the model of Villaroel et al. This is due to the effect of increased solute concentration at the membrane which compensates exactly for the effect of the sieving coefficient. This effect is overlooked in the Villaroel et al. model which assumes well mixed blood and dialysate compartments. Comparison with in vitro clearance measurements for urea, creatinin, vitamin B12, and myoglobin (16,000 daltons) supports this observation even when diffusion dominates as in the case of clinical conditions for hemodiafiltration. An empirical correlation for the overall clearance valid for all solutes and blood flows between 200 and 500 ml/min is found to be K = KD + 0.43 QF + 8.3 x 10(-3) Q2F when clearances and QF are in ml/min.