Abstract: The electronic catheter displacement sensor comprises a displacement sensing roller and an idler roller between which a catheter can be advanced or retracted, circuitry for sensing the rotation of the displacement sensing roller, circuitry for determining the direction in which the displacement sensing roller is rotated, and circuitry coupled to said rotation sensing circuitry and to said direction sensing circuitry for generating an output signal for transmission to a visual display device for displaying the distance the catheter has been advanced or retracted. The rollers can be mounted in a sensor unit separable from the displacement sensor and the sensor unit is preferably disposable.