A major goal of the national aquatic surveys being conducted by the Environmental Protection Agency in partnership with the states and tribes is to assess the relative importance, at a regional scale, of stressors that impact aquatic biota. The Wadeable Streams Assessment (WSA) was a prototype of these surveys, and it assessed 8 individual water-chemistry, physical-habitat, and landuse stressors in 2 ways. First, the WSA estimated the total length of streams in a region that was deemed to be in poor condition for each stressor considered separately. Estimates of stressor extent describe the prevalence and regional breadth of each stressor's potential effects. Second, the WSA estimated each stressor's relative risk, a measure widely used in human epidemiology. Relative risk estimates a stressor's association with biota in terms of the likelihood that poor stressor conditions and poor biota conditions co-occur in a region's streams. We describe how the population attributable risk, also borrowed from epidemiology, combines extent and relative risk into a single, overall measure of a stressor's regional effect. The attributable risk of a stressor is the % reduction in the regional extent of poor biological condition (measured here by the macroinvertebrate index of biotic integrity [MIBI]), that presumably would result from eliminating that stressor. Under the attributable risk assumptions, for example, the WSA data imply that the nationwide extent of poor MIBI conditions would be reduced by an estimated 26% if excess total P were eliminated as a stressor, but only by 3% if excess salinity were eliminated. We also illustrate the attributable risk for the combined effects of multiple, correlated stressors. Last, we discuss how best to interpret and apply attributable risk estimates.