An expandable atherectomy device (50, 70, 80, 90, 300) comprising: a flexible catheter tube; a guidewire over which the atherectomy device is advanceable and which is insertable into a blood vessel until its distal end is adjacent to a site of atheromatous material, said guidewire being introducible through said catheter tube; a rotatably motor-driven flexible hollow shaft (13, 313) that is slidable over said guidewire and is coaxial with the longitudinal axis of said guidewire; an expandable cutting unit having two longitudinally separated ends, wherein said cutting unit, when expanded, is eccentrically rotatable about the longitudinal axis to cut and remove atheromous material from said blood vessel; an actuator which is operable to induce selective expansion of said cutting unit; and said hollow shaft comprising inner (16, 16A; 314) and outer (14, 14A; 94; 316; 1014) tubular portions that are simultaneously rotatable by means of the motor while one of said inner and outer tubular portions slides over the other in a direction substantially parallel to the longitudinal axis in response to an actuated action of said actuator, wherein said cutting unit (19, 79, 89, 99, 1019) is expandable in response to said actuated action while causing said two separated ends (20, 21; 81, 83) of said cutting unit to be brought closer together, wherein a first of said two ends is connected to a distal end of said inner tubular portion and a second of said two ends is connected to said outer tubular portion, characterized in that said inner and outer tubular portions are radially interconnected while rotating simultaneously by means of a pin attached to said inner tubular portion and by means of a widthwise edge of a window formed with said outer tubular portion which has a width only slightly more than said pin. a flexible catheter tube; a guidewire over which the atherectomy device is advanceable and which is insertable into a blood vessel until its distal end is adjacent to a site of atheromatous material, said guidewire being introducible through said catheter tube; a rotatably motor-driven flexible hollow shaft (13, 313) that is slidable over said guidewire and is coaxial with the longitudinal axis of said guidewire; an expandable cutting unit having two longitudinally separated ends, wherein said cutting unit, when expanded, is eccentrically rotatable about the longitudinal axis to cut and remove atheromous material from said blood vessel; an actuator which is operable to induce selective expansion of said cutting unit; and said hollow shaft comprising inner (16, 16A; 314) and outer (14, 14A; 94; 316; 1014) tubular portions that are simultaneously rotatable by means of the motor while one of said inner and outer tubular portions slides over the other in a direction substantially parallel to the longitudinal axis in response to an actuated action of said actuator, wherein said cutting unit (19, 79, 89, 99, 1019) is expandable in response to said actuated action while causing said two separated ends (20, 21; 81, 83) of said cutting unit to be brought closer together, wherein a first of said two ends is connected to a distal end of said inner tubular portion and a second of said two ends is connected to said outer tubular portion, characterized in that said inner and outer tubular portions are radially interconnected while rotating simultaneously by means of a pin attached to said inner tubular portion and by means of a widthwise edge of a window formed with said outer tubular portion which has a width only slightly more than said pin. wherein said cutting unit (19, 79, 89, 99, 1019) is expandable in response to said actuated action while causing said two separated ends (20, 21; 81, 83) of said cutting unit to be brought closer together, wherein a first of said two ends is connected to a distal end of said inner tubular portion and a second of said two ends is connected to said outer tubular portion, characterized in that said inner and outer tubular portions are radially interconnected while rotating simultaneously by means of a pin attached to said inner tubular portion and by means of a widthwise edge of a window formed with said outer tubular portion which has a width only slightly more than said pin.