The prevalence of horn aberrations in Dall's sheep from the Kluane Lake area of Yukon Territory observed during July 1982 represented 1% of the total population and 7% of rams 6 yr or older. Ewes were not considered in these percentages because they were too difficult to inspect by aerial survey. When these data were combined with other data collected from 1977 through 1981, the prevalence equalled 2.4% of the total population and slightly exceeded 16% in mature rams (≥6 yr). The anomaly followed necrosis of the terminal region of the horn core and the sequestering of portions of the core within the sheath as the sheath continued to grow. Sheath that was produced after the core was anatomically altered resulted in abnormal growth patterns of the horn. Rams with aberrant horns could not maintain homeostatic temperatures within horn cores when horns were experimentally exposed to −80 C for 30 min. Histologic examination of superficial and cornual vascular systems did not reveal any structural alterations that would restrict blood flow within cores of affected horns. Examination of museum specimens consisting of 130 skulls from rams and 81 from ewes collected from Alaska, USA, Yukon Territory, Canada, and Northwest Territories, Canada, established only one ewe and no rams with the horn aberration. The skull was from a ewe and had both horns affected and was collected from the Joe River Drainage, Yukon Territory in 1912.