ney. Responses to mild cold stress are predicted by different individual characteristics in young and older subjects. J Appl Physiol 101: 1607-1615, 2006. First published August 3, 2006 doi:10.1152/japplphysiol.00717.2006.-Older individuals' ability to maintain core temperature during cold stress is impaired; however, the relative importance of individual characteristics that influence this response are unknown. The purpose of this study was to determine the relative influence of individual characteristics on core temperature and tissue insulation (I t) during mild cold stress. Forty-two young (23 Ϯ 1 yr, range 18 -30 yr) and 46 older (71 Ϯ 1 yr, range 65-89 yr) subjects, varying widely in muscularity, adiposity, and body size, underwent a transient cooling protocol during which esophageal temperature (T es) was measured continuously and I t was calculated using standard equations. Multipleregression analyses were performed to determine predictors of T es and It, and standardized regression coefficients were analyzed to determine the relative influence of each predictor. Candidate predictors included age, sex, weight, body surface area, body surface area-to-mass ratio, sum of skinfolds, percent fat, appendicular skeletal muscle mass, and thyroid hormone concentrations (triiodothyronine, thyronine). The sum of skinfolds explained 67% (P Ͻ 0.01) of the T es variance in young subjects vs. 2% (P ϭ 0.30) in older subjects. Conversely, appendicular skeletal muscle mass explained a greater portion of the variance in older subjects for both T es (older: 28%, P Ͻ 0.01; young: 8%, not significant) and It (older: 46%, P Ͻ 0.01; young: 17%, P Ͻ 0.01). The Tes residual variance was considerably larger in older subjects (59 -72% vs. 14 -42% in young subjects), possibly due to varying rates of physiological aging. These results suggest that the relative influence of individual characteristics changes with aging. core temperature; tissue insulation; body composition; aging EPIDEMIOLOGICAL EVIDENCE SUGGESTS that older individuals are more susceptible to hypothermia than young individuals, with Ͼ50% of deaths due to hypothermia occurring in those aged Ͼ65 yr, an age cohort comprising only ϳ12% of the US population (8). Cross-sectional studies confirm that aged individuals have a relative inability to maintain core temperature