A reinforced flexible x-ray film cassette includes two cassette parts which are connected at one side by a hinge and which are cylindrically curved as considered in a plane normal to the hinge. Two circumferentially extending ribs are provided on one of the cassette parts and a single circumferentially extending rib is provided on the other cassette part, the single rib being received between the two ribs in the closed position of the cassette to form a light barrier therewith and being in force-transmitting engagement therewith to reinforce the cassette against bending or twisting. The cassette part which constitutes the bottom includes a foil of ferromagnetic material floatingly supported on an elastic peripheral strip and an intensifying shield supported on this foil. The cassette part which constitutes the top includes a magnetic plate supported on elongated projections of the cassette wall and supporting another intensifying shield. In the closed position of the cassette, the magnetic plate attracts the ferromagnetic foil to thereby eliminate air inclusions between the intensifying shields and the x-ray film interposed therebetween. The hinge includes two rails interconnected by a flexible, part-cylindrical connecting portion. Each of the rails of the hinge has a recess which receives a correspondingly configurated projection of the frame of the respective cassette part.