Analysis of future Japan Meteorological Agency atmospheric general circulation model (JMA AGCM) based climate scenarios for the Mekong River basin (MRB) indicates that annual mean precipitation will increase in the 21st century (2080–2099) by 4Ð2% averaged across the basin, with the majority of this increase occurring over the northern MRB (i.e. China). Annual mean temperatures are also projected to increase by approximately 2Ð6°C (averaged across the MRB). As expected, these changes also lead to signi?cant changes in the hydrology of the MRB. All MRB subbasins will experience an increase in the number of wet days in the ‘future’ and, importantly for sustainable water resources management and the mitigation of extreme events (e.g. ?oods and droughts), the magnitude and frequency of what are now considered extreme events are also expected to increase resulting in increased risk of ?ooding, but a reduction in the likelihood of droughts/low-?ow periods—assuming water extraction is kept at a sustainable level. Despite the fact that the climate change impact projections are associated with signi?cant uncertainty, it is important to act now and put in place policies, infrastructure and mitigation strategies to protect against the increased ?ooding that could occur. In addition, despite this study indicating a decrease in the number of ‘low-?ow’ days, across most of the MRB, further analysis is needed to determine whether the reduction in low-?ow days is enough to compensate for (and sustain) the rapidly increasing population and development in the MRB. Copyright ? 2008 John Wiley & Sons, Ltd.