Improved methods are needed for dose quantification in dialysis because the kinetic modelling based on blood side urea concentrations, as currently used, is subject to several theoretical and practical problems. Since the treatment induces a disequilibrium, it is necessary to make assumptions about the distribution and exchange of urea within the body. A new method is presented which avoids these problems. The amount of urea removed is determined from continuous measurements in the spent dialysate. It is shown that the relative dialysis efficiency K/V can be calculated from the mass removal rate in relation to the mass remaining in the body, which is also determined from the measurements. As a by-product the total mass of urea in the body before the treatment is obtained. This can be used together with the blood concentration to calculate the urea distribution volume, which might be used as an additional clinical parameter. The new method was tested in three in vitro treatments of a container with a known amount of urea added to a known volume of dialysate. The calculated dose KT/V and initial urea mass differed from the true values by less than 3%.