Abstract: A tamper-proof container closure device having a closed cap member and a crimping ring. The closed cap member is internally threaded and has a flared annular skirt extending around its base. The crimping ring is snapped into a retained position on the flared annular skirt before the cap is screwed onto a container. After the cap is screwed onto a container, the crimping ring is deformable radially inward around a suitable boss at the container opening onto which the container closure device is installed, thereby drawing the flared annular skirt into sealing engagement with the periphery of the boss and preventing removal of the cap without first visibly tearing off the ring.