Abstract: A multistaged telescope boom, where a hydraulic unit, comprising a piston and a cylinder, is arranged between successive, telescopic arms, comprises a hydraulic system connecting to said hydraulic units for the operation thereof and which is designed to force the hydraulic units of at least the two innermost arms for filling a cylinder chamber of one of these cylinders (9, 10) at the time starting from that of the innermost arm and outwardly in the order of the arms when extending the boom and draining said cylinder chambers in the opposite order when retracting the boom. At least the hydraulic unit of said innermost arm is provided with an arrangement adapted to isolate the cylinder chamber of that first cylinder from communication with said hydraulic system when the cylinder is fully extended and reestablish said communication upon fully retraction of the cylinder next to said first cylinder.