During 2008–2010 we investigated the ecology of the Endangered dhole or wild dog Cuon alpinus in Khangchendzonga Biosphere Reserve in the Eastern Himalaya in Sikkim, India. We conducted camera trapping (n = 71 sites, 6,278 camera-days) and sign surveying along trails (n = 24; 629.43 km of effort) to assess the relative abundance, distribution and activity pattern of the dhole. Morphological characteristics evident in the 61 camera-trap photographs indicate that the dhole population in the Reserve may be the rare and genetically distinct subspecies C. alpinus primaevus. We detected dholes over a wide elevation range (2,501–4,100 m) that encompassed the upper temperate, subalpine, and alpine scrub zones. Dholes were diurnal, with peak activity at 08.00–10.00. Analysis of 41 scats indicated a diet comprising mainly mountain ungulates, rodents and pikas Ochotona sp. Although the frequency of occurrence of rodents was highest (32%) in the scats, 98.7% of the total biomass consumed was of mountain ungulates. Historical reports (1888–1894) indicated that the dhole was formerly abundant in Sikkim but was hunted to meet the high demand for its alleged medicinal properties. With no information on the status of the dhole in Sikkim for over a century, our study suggests that the species is now rare in the Reserve. To aid the conservation of the dhole and its main ungulate prey species extensive research and monitoring are required in the Reserve and elsewhere in the Eastern Himalaya.