A bellows for hydraulic, hydropneumatic, or pneumatic piston-cylinder units, especially for vibration dampers or MacPherson strut units for motor vehicles, is located between the end of a piston rod and a cylinder, and covers the part of the piston rod which projects out from the cylinder to provide protection against dirt and/or damage. An interior space of the bellows undergoes a change in volume as a result of the relative movement of the piston and cylinder. The end area of the piston rod distal from the cylinder includes at least one flow connection leading to the interior of the bellows. The flow connection allows an incoming and/or outgoing flow of air. The end area of the piston rod, i.e., the end which passes through a mounting bearing connecting the piston-cylinder unit to the body of the vehicle, is provided with a flow connection connecting the interior space of the bellows to the atmosphere.