There is disclosed a novel power adjustable guide holder or support for a bar and tube straightening machine. A lower guide holder or support assembly includes an elongated strong base unit comprising a centrally hollow base plate and an integral elongated housing supporting operating parts. A power adjustable cover is disposed over the elongated housing and includes on the top thereof a tapered skewed groove which receives and retains against accidental removal a guide, the upper edge of which is adapted to be engaged by a bar, tube, or the like, being straightened by the machine. The over also prevents scale, coolant, and other foreign matter from falling into and jamming the mechanical operating structure within the guide holder assembly. Hydraulic motors within the housing are selectively actuatable to rotate vertical threaded shafts carrying rider nuts for raising and lowering the ends of the cover to adjust the positions of the retained guides. The elongated base unit is a structural member of sufficient strength to resist the turning forces of a bar as it goes through the machine on top of the guide and yet is narrow enough so that the guide holder does not interfere with the rolls of the machine. The top of the cover is wide enough to permit the skewing of the guide retaining groove, thereby giving additional clearance in respect to the rolls.