Abstract: A replacement window frame assembly includes bottom, sectioned-top, and first and second side cap members for placement over an existing window frame in a building from which the windows and intermediate support members have been removed. The cap members are made from a uniform aluminum extrusion which has a front lip to extend over the outside of the existing frame, and a lower depending lip, designed to rest on the window sill, or abut against the side and top of the frame opening in which the original frame is placed. The cap members are placed over the existing window frame to form a flat mounting surface for the installation of a replacement window. The entire replacement may be effected from the outside of the building, and is accomplished without destroying or damaging any of the interior or exterior finishes of the building.