A new system for ultrafiltration control during hemodialysis is described. The apparatus consists of a computer operated system of load cells that register variations in weight of the outlet dialysate versus inlet dialysate. Once the weight loss of the patient has been established, the gravimetric control operates on the dialysate circuit to obtain the transmembrane pressure adequate to achieve the desired ultrafiltration rate and patient weight loss. The system can be used as a complete dialysis machine or as a separate module that can be adapted to any standard dialysis machine. This module was tested in more than 220 dialysis sessions, using different membranes and ultrafiltration rates. The difference between the scheduled and the real weight loss was always less than 100 g at the end of the dialysis session. The number of technical interventions required were few, as was the rate of complications related to the system. The system is safe and reliable and offers a low cost opportunity to improve dialysis tolerance by accurate and progressive ultrafiltration during the session.