Abstract: A longitudinally controllable adjustment device, in particular a gas spring partially filled with a fluid has a piston disposed on a piston rod. In order to achieve simple manufacture and assembly and high operational safety, a valve housing for receiving a valve is provided on the piston rod. Further, the piston axially covers to total valve. The piston is secured to the piston rod by deformation, which simultaneously results in a pre-stressing of a seal sealing in four directions. The piston rod with the valve is compressed against a seal in the piston, which seal sealingly abuts on an annular bottom of a receptacle, on the receptacle, on the valve and on the valve body.