An atherectomy catheter includes a catheter body having a cylindrical housing at its distal end. The cylindrical housing includes an elongate cutting aperture on one side thereof and a mechanism for severing atheroma which enters within the opening. In a first embodiment, the cutting mechanism comprises a circular cutting blade which is advanced over an elongate guide member which defines a path for the blade. Use of the guide means helps assure that the cutting blade will not be lost from the housing during use. In a second embodiment, a rotatable helical cutting blade is mounted within the housing. In that particular embodiment, the helical cutting blade can serve as the guide member for the circular cutting blade.