The invention concerns a process for the manufacture of a flame-resistant elastic polyurethane foam which has combustion characteristics which satisfy the severest requirements and fire tests and whose properties remain almost unchanged over a long period in use. The process uses a foam reaction mixture with a polyol and a polyisocyanate plus a proportion of expanded graphite in platelet form as the flame-retardant, the graphite platelet size being the same order of magnitude as the size of the cell walls in the foam formed. The graphite is mixed with the polyol reaction component and incorporated in the effervescing foam in such a way that the platelets form at least part of the cell walls.