Abstract: There is described an improved apparatus and method of treating a section of unlined well bore comprising the steps of establishing a flow path from the top of the well bore to a location opposite the section of unlined well bore to be treated, pumping an erosive fluid through the flow path at a predetermined rate and pressure, directing a stream of the erosive fluid against a surface of the section of well bore to be treated to cause the initiation of a cut thereinto, and moving the stream of erosive fluid past a length of the surface to be treated to extend the cut formed therein in the direction of movement of the stream.