Background/Question/Methods Many harvested species have recovered from the exploitation of the early 1900's, but limited data exist that link these changes to increases in disease. We analyzed how changes in elk (Cervus elaphus) populations and aggregation patterns may be affecting the dynamics of brucellosis in the Greater Yellowstone Ecosystem (GYE) using statistical and mechanistic modeling approaches. Cattle herds in Montana, Idaho and Wyoming have been infected by brucellosis over the past 5 years, and the available data suggest that elk are the most likely source. The historically low seroprevalence of brucellosis in elk populations fostered a consensus that Brucella abortus is not self-sustaining in elk populations that do not concentrate on supplemental feedgrounds or share winter range with bison. This belief was also supported by the lack of brucellosis in elk populations outside the GYE. Here we argue that the status of free-ranging elk as a viable reservoir for brucellosis may be changing as their populations increase in number and density. Results/Conclusions Brucellosis prevalence in Wyoming elk increased from 0-6% in 1991/92 to 7-18% in 2006-08 in four of five herd units. These levels of prevalence were comparable to some of the winter feedgrounds where elk are artificially aggregated and prevalence varied spatially from 6-36%. Increases in seroprevalence corresponded with large elk populations, with one exception. Model simulations suggest that these levels of seroprevalence cannot be sustained by dispersal from the supplemental feedgrounds, but are probably due to enhanced elk-to-elk transmission on native winter ranges resulting from larger winter aggregations. Therefore, unfed elk populations may now be competent reservoir hosts for Brucella abortus independent of the Wyoming feedgrounds. Some of Montana's elk populations were five to nine times larger in 2007 than in the 1970's with some aggregations that were comparable to the Wyoming feedground populations, placing them at greater risk for future disease outbreaks. Addressing the unintended consequences of successful conservation efforts in the ‘New West' is complicated by limited hunter access to private lands that places many ungulate populations out of administrative control. Agency-landowner hunting access partnerships and the protection of large predators are two management strategies that may be used to target high ungulate densities in private refuges and reduce the current and future burden of disease.