In the U.K., a full recovery of Hen Harrier Circus cyaneus breeding numbers is prevented by illegal culling by some gamekeepers who fear the species threatens the future of grouse moors. This study's main purpose was to estimate how many more Hen Harriers there would be in the U.K. if this culling were to cease. A review of 33 studies of the Hen Harrier in nine countries showed that the densities of nesting females decreased strongly as the size of the areas studied increased and also that such information could not be used to estimate the potential density over a large area, such as the U.K. Correcting for the size of the areas studied showed that the potential density of nesting Hen Harriers on grouse moors could be predicted from studies overseas. Estimates of the densities of Hen Harriers in occupied areas globally were therefore used to estimate the density appropriate for the U.K. If all potential habitats were occupied, present numbers could more than double, to an estimated 1660 nesting females. This estimate represents an average of one nesting female per 2 5 km2 of habitat, a density which would cause little or no significant economic damage on grouse moors. However, because Hen Harriers tend to aggregate, they would not spread out evenly but would nest in relatively high densities on a number of moors. The economic impact on Red Grouse Lagopus lagopus would not be a function of overall numbers, rather it would arise from the uneven dispersion of nesting Hen Harriers.