This study investigates the Mekong River basin hydrologyfor the 1980–2000 period using a grid-based distributedhydrological model called Yamanashi Hydrological Model (YHyM). The performance of the model is evaluated using data observed at different locations and the results justify the physical soundness of the model. The seasonal variations of climatic and hydrological characteristics of the basin such as soil moisture, ground water saturation de?cit, runoff, precipitation, evapotranspiration, etc. are analysed. On the basis of the simulated results, it is noticeable that there is no signi?cant trend in the precipitation, discharge, or soil moisture state of the basin during the simulated period, though there are some seasonal variations which seem to be natural. However the analysis on the precipitation elasticity (E) of the river ?ow shows that the E values for all sub-basins are greater than unity, which indicates that x% change in annual precipitation can cause >x% change in annual river ?ow. Further the basin hydrological responses are analysed for a long term synthetically-induced drought the results of which show the signi?cance of the base ?ow of the Mekong River basin. Copyright ? 2008 John Wiley & Sons, Ltd.