Abstract: A folding sofa bed frame having head, intermediate and foot bed sections pivotally connected end-to-end and constrained by a linkage arrangement to move in a predetermined pattern between a folded position and a horiziontally extended position. The linkage arrangement includes a bracing link with a stud which, in the extended position, engages the rearward side of an upright support link by which the structure is mounted within the sofa bed frame, to act as a stop member against undesired accidental upward tilting of the structure when weight or force is placed against the head section. A transverse mattress support bar is affixed to a pivoting control link also operated by the bracing link for movement of the mattress support bar into a retracted position closely adjacent the intermediate section when the structure is folded and a dropped position spaced below the intermediate section when the structure is extended, thereby out of interference with comfortable sleeping use of the bed frame.