We present variations of CO2 in the tropopause region obtained by frequent in situ measurements aboard commercial aircraft. The data were obtained from a total of 373 flights between Japan and Europe during the period November 2005 to September 2007. The local phase and amplitude of the CO2 seasonal cycle varied with distance from the tropopause. In the upper troposphere and in the region just above the dynamical tropopause, a strong seasonal cycle with a springtime maximum and a relatively sharp minimum in July was observed. In the region bounded by potential temperatures 10 K to 30 K above the extratropical tropopause, no significant seasonal cycles were found. In the region greater than 30 K from the tropopause (i.e., at higher altitudes), sharp CO2 increases in summer followed by gradual decreases were found, resulting in a slightly increasing seasonal cycle amplitude with distance from the tropopause. The observed CO2 distributions also showed that CO2 isopleths followed the tropopause during the winter and spring, whereas in the summer they tracked potential temperature surfaces crossing the tropopause. The observed seasonal variation in CO2 suggests that the lowermost stratospheric region is influenced by a combination of (1) fast meridional transport of high CO2 from the tropical troposphere in the summer, (2) active subsidence of low CO2 from higher altitudes in the spring, and (3) relatively weak vertical mixing near the tropopause.