Methods for calculating required dialysis time to maintain certain blood urea nitrogen levels predialytically thus far have neglected the influence of ultrafiltration during hemodialysis and of weight changes in the interdialytic period. We therefore have developed a new algorithm for individual dialysis prescription based on urea kinetics which further optimizes accuracy of dialysis prescription by considering these two factors. The accuracy of this new method was proven in a prospective follow-up of 3 patients during some 9 consecutive hemodialysis session. It was found that there was a very close relation between the measured and the predicted values of blood urea nitrogen, the difference being only due to variations in dietary protein intake as the patients were kept on liberal food intake. The new algorithm is easily implemented into a pocket calculator and thus can be used under arbitrary dialysis conditions.