AbstractMany species on endangered species lists such as the IUCN Red List (RL) are categorized using demographic factors such as numbers of mature individuals. Genetic factors are not currently used in the RL even though their explicit consideration, including effective population size (Ne) and expected heterozygosity-loss (H-loss), could improve the assessment of extinction risk. Here, we consider the estimation ofNeandH-loss in the context of RL species. First, we investigate the reporting of number of mature individuals for RL Endangered species, which is needed to estimateNeandH-loss. We found 77% of species assessments studied here did not report methods used to estimate the number of mature adults, and that these assessments rarely report other important determinants ofNe(e.g., sex ratio, variance in family size). We therefore applied common rules of thumb to estimateNe, and found thatNewas likely < 50 for at least 25% of the 170 RL Endangered species studied here. We also estimated mean expectedH-loss for these species over the next 100 years, and found it to be 9–29%. These estimates of highH-loss and lowNesuggest that some species listed as Endangered likely warrant listing as Critically Endangered if genetic considerations were included. We recommend that RL and other assessment frameworks (i) report methods used for estimating the number of mature adults, (ii) include standardized information on species traits that influenceNeto facilitateNeestimation, and (iii) consider using concepts likeNeand heterozygosity-loss in risk assessments.