Analysis of northern hemisphere mid-latitude temperature data at 30 mbar over the last 20 years suggests a long-term negative trend of about -0.24 K/decade when all observations between 10¿N and 90¿N are considered. This stratospheric cooling which could tentatively be attributed to the increase of the carbon dioxide amount in the stratosphere is in qualitative agreement at 30 mbar with a model calculation of the historical evolution of temperature and chemical composition in the middle atmosphere.