The Microwave Limb Sounder (MLS), an instrument on the Upper Atmosphere Research Satellite (UARS), measures water vapor in the upper troposphere, with best sensitivity at the standard UARS level at 215 hPa. In this paper, we analyze the MLS observations with a view to characterizing the temporal and zonal variations of upper tropospheric water vapor between 20 ¿N and 20 ¿S. Time series of water vapor throughout the tropics show a strong annual cycle with maximum amplitude at 20 ¿N and 90 ¿E. An intraseasonal cycle with a period of 30--85 days is evident over the Western Pacific at latitudes from 10 ¿N to 20 ¿S. The cycle is associated with eastward propagating disturbances of zonal wavenumbers 1--2, suggesting that this intraseasonal cycle is related to the Madden-Julian oscillation. ¿ 1998 American Geophysical Union