An eccentric screw pump comprising an outer part (2) and an inner part (3) therein, one of the parts (2, 3) being driven rotatably and the other part (2, 3) being able to move eccentrically relative to the other part (3, 2). The screwthreads (5a, 5b) of the outer part (2) extend angularly over less than an entire helix along the axial length (L) of the part, so that during operation pumping chambers that are open to both ends are created, through which sudden pressure relief takes place.