Abstract: A process and an apparatus for monitoring the progress of haemodialysis by extra-corporeal measurement of the haematocrit are presented. The process comprises the steps of:calculating at a given time the variation of a function of the haematocrit over a short period, on the one hand, and over a medium period preceding this time, on the other,comparing the values of these variations,the result of this comparison is a first indicator of the progress of the haemodialysis.