Lidar measurements of the worldwide movement of stratospheric aerosols produced by the 18 May 1980 eruption of Mount St. Helens are described. Ground-based and airborne measurements show that the layers below 20 km produced by this eruption moved in an easterly direction while those above 20 km moved in a westerly direction. The effluent at jet stream altitudes of 10 to 12 km circled the globe in about 16 days and the effluent at 23 km (the highest altitude recorded) circled the globe in about 56 days. Mass calculations, using backscatter-to-mass conversion models, indicate that approximately 0.5 X 106 metric tons of new stratospheric material were produced by this eruption. Even though this represents a 200% increase in North-ern Hemispheric aerosol, no significant long-term atmospheric temperature change should occur.