Abstract: This invention relates to a helmet for cyclists, skateboard runners and roller skaters. It comprises a shell of flexible and hard plate-shaped material and a damping means attached inside the shell. The shell in its plane has the form of an I, with extended cross-pieces (3) and with tips (3) attached radially between the same, which tips extend from the connecting between the cross-pieces and the stem of the I. About at the center of said stem lateral tips are attached to both sides. The cross-pieces and tips are provided with attachment means in such a manner, that the free ends of one cross-piece after the bending of the stem can be connected to the free ends of the other cross-piece. The free ends of the tips can be attached to suitable holders on the lateral tips.The cross-pieces, tips and lateral tips can be designed as bands, and the attachment means be arranged so that the width and size of the shell can be adjusted.