A window and door jamb adjustment system for accurately installing windows and doors and providing for later adjustment of the same. The window and door jamb adjustment system includes a support member attachable to a building frame, an alignment member slidably attached to the support member and attachable to an extension jamb, a door or window, and a threaded shaft attached to the support member and threadably engaging the alignment member. As the threaded shaft is rotated, the rotational motion is converted to linear motion of the alignment member. As the alignment member is moved with respect to the support member, the extension jamb, a door or window, is correspondingly moved inwardly or outwardly for proper alignment with plasterboard.