Abstract: Apparatus for monitoring flow of a dialysis fluid through a dialyzer is disclosed including a computer for calculating the flow rate of the dialysis fluid into and out of the dialyzer based on the formula P-P.sub.O =k*Q.sup.n in which P is the pressure in the dialysis fluid between a throttle and a pump either upstream or downstream of the dialyzer, P.sub.O is the pressure in the dialysis fluid on the opposite side of the throttles from the pump, k is a characteristic coefficient for the throttle, Q is the flow of the dialysis fluid through the throttle, and N is a characteristic exponent for the throttle, and the apparatus includes microprocessor for calculating the values of k and P.sub.O prior to use and for calculating the value of P.sub.O during use of the dialyzer. Methods for carrying out such monitoring are also disclosed.