In this study, we examine the water vapor transport over the entire Asian monsoon region using the FGGE III-b data of the European Centre for Medium Range Weather Forecasts (ECMWF) and the Geophysical Fluid Dynamics Laboratory (GFDL). Effort is made to explore the possible effect of the 30–50 day oscillation on water vapor transport. We apply the empirical orthogonal function (EOF) analysis to the potential and stream-function of the vertically integrated water vapor transport over a region (25°S–60°N, 40°E–130°W). It is found that the water vapor transport over the Indian and East Asian monsoon regions are related to each other through their interactions with the eastward propagating 30–50 day oscillation. Several occurrences follow the 30–50 day oscillation of the Indian monsoon westerlies in a coherent manner: The east-west shifting of the water vapor convergence center over the Asian monsoon regions; the north-south movement of the North Pacific convergence zone (NPCZ); and the merging of the water vapor flux transported by the Indian monsoon westerlies and the Pacific anticyclone trade winds over the South China Sea. The development of the East Asian monsoon is characterized by the north-south movement of water vapor fronts, such as the Mei-Yü and Baiu fronts, associated with the NPCZ. This study reveals that this north-south movement is caused by the eastward propagation of the 30–50 day mode. We also apply the EOF analysis to the global potential function of the water vapor transport between 45°S and 45°N over the entire FGGE year. It is suggested that the phase relationship between the annual cycle and the 30–50 day mode of the potential function of the water vapor transport can be used to explain the development of the large-scale environment for the Indian monsoon over its entire life cycle. It is also inferred from the time lag between onsets of the Indian and East Asian monsoon that the two monsoons may be induced by the same mechanism, namely, the interaction between the annual cycle and the 30–50 day low-frequency mode.