Abstract: An adjustable helmet, particularly a cyclist's helmet having at least one longitudinal spar which is intended to be positioned above the parting, two end pieces one of which is intended to be positioned above the forehead and the other one above the back of the head. Guides between the end pieces and the longitudinal spar which are aligned approximately in parallel with the parting. Two lateral elements which are intended to be positioned above the temples, guides between the end pieces and the lateral elements which are aligned obliquely to the parting, and an adjusting and/or locating device for the relative position of the longitudinal spar, end pieces, and lateral elements.