The effect of the Madden-Julian Oscillation (MJO) on the Northern Hemisphere wintertime stratospheric polar vortex is evaluated using a meteorological reanalysis data set and a modern atmospheric general circulation model. The MJO influences the tropospheric North Pacific; and in particular, it modulates the heat flux that is in phase with the climatological planetary waves in both the troposphere and stratosphere. The phase of the MJO in which convection is propagating into the tropical central Pacific leads to a weakened vortex, while suppressed MJO convection in this region is associated with a stronger vortex. Subsequently, the MJO modulates the phase of the tropospheric North Atlantic Oscillation (also known as the Arctic Oscillation or the Northern Annular Mode). While the responses in the model and in the reanalysis data differ in some respects, they both indicate that the MJO can remotely impact the extratropical tropospheric circulation via the stratosphere.