Study region: The middle hilly region of the Koshi River Basin in Nepal. Study focus: Assessment is made of the hydrological regime of the basin under climate change. Results from two Regional Climate Models (PRECIS-HADCM3Q0 and PRECIS-ECHAM05), based on IPCC-SRES A1B scenario, were bias corrected against historical gauged data. Hydrological impact simulations were conducted using SWAT model. Design flood estimation was done after extreme value analysis based on annual flow maxima. New hydrological insights for the region: The study found that climate change does not pose major threat on average water availability. However, temporal flow variations are expected to increase in the future. The magnitude of projected flow for given return periods, however, strongly depends on the climate model run considered. The ECHAM05 results show higher flow changes than those estimated from the HADCM3 outputs. A relation was derived to estimate projected flood flow as a function of return period and flow estimated from historical series. Amidst the uncertainties, these predictions provide reasonable insight for re-consideration of design standards or design values of hydraulic structures under climate change.