Estimates of the impacts of climate change on streamflow generally have not included concurrent effects of urbanization. A statistical analysis of historical streamflow, climate, and population data for 39 urbanizing and 21 nearby rural basins in four regions of the United States was used to estimate the future effects of climate change and urbanization on mean annual streamflow. Basins were located generally at lower elevations where streamflow was dominated by rainfall rather than snowmelt. Rural basins showed predicted average changes in mean annual streamflow ranging between +24 and -49% for the specific climate change scenarios tested (precipitation changes -20 to +20%, temperature changes 0 ¿C to +4 ¿C). Urbanization increased mean annual streamflow in rough proportion to average cumulative changes in population density on the basins, equivalent to an average flow increase of 103% with complete watershed urbanization. Urbanization also appeared to reduce the sensitivity of mean annual streamflow to temperature changes compared to mean flow response on rural basins. No significant regional differences in mean flow response to climate change and urbanization were found. Despite the uncertainty in predicting future streamflow with models based upon past records, urbanization appears potentially capable of significantly offsetting flow declines or augmenting flow increases caused by climate change. ¿ 2000 American Geophysical Union