Commonly used measures of the impact of an exposure on disease are inadequate for assessing the potential benefit of community-based efforts to prevent disease caused by the exposure in question. Because relative measures of effect, including the risk ratio, odds ratio, and population attributable risk (PAR) do not account for the absolute risk of disease, they lack the most crucial element for evaluating the opportunity for prevention of disease caused by the exposure. Attributable community risk (ACR), defined as the difference between the crude risk (or overall risk in the population) and the risk in the unexposed, better captures the potential impact of a prevention effort. PAR can be expressed as the proportion of the disease incidence in a population that is attributable to the exposure. ACR, by contrast, is the proportion of the population that is affected with the disease due to the exposure. Therefore, unlike PAR, risk ratio or odds ratio, ACR is useful for comparing the potential benefit of programs aimed at the prevention of different diseases. For example, ACR provides a useful comparison of the potential benefits of efforts to prevent breast cancer and to prevent ovarian cancer among women with a BRCA1 or BRCA2 mutation.