The midwinter temperature changes of the mesosphere and stratosphere are described by means of Satellite Infrared Spectrometer and Selective Chopper Radiometer data, rocketsondes, and rocket grenade data, which show that the so-called stratospheric midwinter warmings extend at least into the upper mesosphere. Temperature changes of opposite sign take place at the same time at different levels, probably as a result of vertical motion. The event begins around a very high stratopause, ∼60 km, which descends 20 km within several days while the warming intensifies. At the same time the upper mesosphere and lower stratosphere cool. When the polar vortex breaks down, the warming reaches the lower stratosphere, the warm stratopause region is destroyed through cooling of the layer between 30 and 60 km, and the upper mesosphere warms. The mean vertical temperature profiles suggest that the upper mesosphere is cold at high latitudes in early winter and again in late winter, and that the warm upper mesosphere observed in late January–early February is associated with the breakdown of the stratospheric polar vortex.