Water vapor in the tropical upper troposphere, above the altitudes ordinarily sampled by radiosonde humidity elements, is important in controlling atmospheric infrared energy loss to space, both by its dominance of infrared transmission and its role in cloud formation. Much of the water vapor arrives in the upper troposphere above about 12 km in plumes associated with cumulonimbus complexes; values at 215 and 147 hPa have recently been estimated from the Microwave Limb Sounder on the NASA Upper Atmosphere Research Satellite. In equatorial regions, time changes of upper troposphere moisture over a three year period, 1991--1994, are closely related to sea surface temperature changes in the eastern tropical Pacific, including seasonal and non-seasonal variations. Maps of the annual and semi-annual components at 215 and 147 hPa show maxima over the eastern hemisphere land masses and the maritime continent, as well as over Central and South America.¿ 1997 American Geophysical Union