Stochastic rainfall runoff modelling is used to derive fllod frequency characteristics under current climate conditions using a semi-emphirical rainstorm model and TOPMODEL. In an application to the River Wye catch,ent in Wales, it is shown that parameter sets can be found that will reproduce both continuous hydrographs and frequency characteristics acceptably. The approach takes account of uncertainty in the modelling process using the GLUE methodology. The uncertainty includes the realisation effect of only having limited observed data available. The inputs to the model are then modified according to future climatic scenarios derived from GCM runs made by the Hadley Centre. It is shown that for all return periods, the uncertainty in flood magnitudes under current conditions is greater than the predicted increase due to climate change. However, within this framework the changing risk of a certain flood magnitude occurring with a certain return period can be realistically assessed.