Abstract: An adjustable door and frame assembly adapted to fit door openings of different sizes. The door is made up of a core piece having a plurality of height and width extension members which may be removably attached thereto to change the dimensions of the door. The door also has vertically adjustable door knob and hinge receiving assemblies. The frame includes a hinge jamb member, a strike jamb member, and a header member and a plurality of frame extension members which may be attached in varying numbers to the frame members to change the height and width of the frame. All the frame members are made up of a door strike member and a facing member with interfitting tongue and groove assemblies that allow the thickness of the frame to be adjusted. Preferably, all parts of the door are made of a plastic material so that the door is lightweight, easy to clean, and free from warping, swelling, splitting, and rotting.