Sieving coefficients (SC) for middle molecular weight (MMW) substances have been found to decrease markedly as filtration rates (Qf) are increased. To best evaluate the newly available Renaflo hemofilter, we studied the effect of increasing filtration rates on the SC of a sample MMW substance (inulin). An in-vitro filtration system was designed to deliver varying Qf from 4.1 to 17.7 ml/min. Serum and filtrate chemistries (urea, creatinine, total protein) were determined. The results revealed no change in sieving for urea or creatinine and a universally low sieving for protein. The sieving coefficient for inulin, however, decreased significantly; from 0.93 at a Qf of 4.1 ml/min to 0.79 at a Qf of 17.7 ml/min (p less than 0.03). These results confirm that the SC of MMW substances do decrease as Qf increase. Despite the decrease found, when compared to previously published data, the Renaflo hemofilter seems to offer a greater degree of middle molecular weight sieving under conditions encountered during CAVH.