Continuous renal replacement therapies have been used increasingly in critically ill patients in nephrologic and intensive care units. These techniques are nevertheless time consuming and lack an easy control of ultrafiltration. We conducted continuous venovenous hemodialysis (CVVHD) on a dialysate delivery machine which ensures a volumetric control of ultrafiltration, and which produces both dialysate and infusate with the on-line technique. This new mode of CVVHD was firstly assessed in a feasibility study and was secondly used in a randomized prospective study whose goal was to establish the usefulness of convection. On-line continuous venovenous hemodiafiltration (OL-CVVHDF) appeared safe and time saving, and allowed the fine control of fluid management in critically ill patients. Convective transfers were achieved by a 2 L/h predilutional infusion and increased urea and creatinine clearances twofold. OL-CVVHDF revealed itself to be a valuable method for the control of ultrafiltration and for efficient convective transfers during CVVHD.