The Vancouver Island marmot (VIM; Marmota vancouverensis) is a critically endangered sciurid endemic to the subalpine meadows of Vancouver Island, British Columbia, Canada. In response to population declines in the 1980s and 1990s, an intensive conservation breeding and reintroduction program was initiated in 1997. This recovery effort has generated significant health-related data from the captive, captive-release, and wild VIM populations, including results from 153 ex situ and 820 in situ mortalities. There were important differences in surveillance between captive and free-ranging populations (captive release and wild). Ex situ marmots exhibited greater longevity, were monitored with greater intensity, were carefully managed during hibernation, and were safeguarded against predation. These factors facilitated prompt recognition, recovery, and evaluation of mortalities. Cardiomyopathy and neoplasia represented the most significant causes of mortality in captives but were not observed in their in situ counterparts, possibly due to reduced longevity, dissimilar risk patterns, or limited opportunities to evaluate intact carcasses. Intact carcasses of free-ranging marmots were mainly recovered in spring and resulted from hibernation-related, postemergent emaciation. Mortalities of free-ranging VIMs often presented as incomplete remains or were inaccessible in burrows, meaning they were evaluated according to on-site field observations. Investigations indicated that the first wild hibernation was a time of substantial mortality for marmots released from captivity. However, captive releases that survived their first winter displayed a low incidence of hibernation mortality in subsequent winters, a success rate similar to that of their wild-born counterparts. Predators, particularly cougars (Puma concolor), represented the most consequential cause of mortality for both captive-release and wild marmots. No specific infectious agents were identified as presenting a generalized threat to in situ or ex situ populations. Chronic obesity in captives, reduced hibernation survival of first-year captive releases, and pronounced, seasonal predation of free-ranging marmots appear to represent important factors influencing the health and recovery of this critically endangered species.