Abstract: The change in body fluid is monitored by placing a pair of electrodes at spaced locations on a body, i.e. a wrist and a diametrically opposed ankle and generating two high frequency alternating currents, one of 1.5 kHz and the other 150 kHz, through the body via the electrodes. Another pair of electrodes is placed in the current path of the first pair of electrodes and connected to a detector for measuring body impedances at the two frequencies. A microprocessor calculates the change in fluid weight of the body according to the empiric formula: ##EQU1## where .DELTA.V=change in fluid weightk=a proportionality constant having a negative valueS=body surface area, based on the patient's weight and lengthZ.sub.1 and Z.sub.2 =impedance at the lower frequency (for example 1.5 kHz), and respectively, the higher frequency (for example 150 kHz) at the second measuring occasionZ.sub.1.sbsb.i and Z.sub.2.sbsb.i =corresponding impedances at the first measuring occasion (intitial values).