Previous retrospective studies report a core body temperature cooling rate of 3 ° C/hr during avalanche burial. Hypercapnia occurs during avalanche burial secondary to rebreathing expired air, and the effect of hypercapnia on hypothermia during avalanche burial is unknown. The objective of this study was to determine the core temperature cooling rate during snow burial under normocapnic and hypercapnic conditions. We measured rectal core body temperature (T_re) in 12 subjects buried in compacted snow dressed in a lightweight clothing insulation system during two different study burials. In one burial subjects breathed with a device (AvaLung 2^TM, Black Diamond Equipment Ltd.) that resulted in hypercapnia over 30 to 60 minutes. In a control burial subjects were buried under identical conditions with a modified breathing device that maintained normocapnia. Mean snow temperature was - 2.5 ±2.0 ° C. Burial time was 49 ±14 minutes in the hypercapnic study and was 60 minutes for all subjects in the normocapnic study (P=0.02). Rate of decrease in T_re was greater with hypercapnia (1.2 ° C/hr by multiple regression analysis, 95 % confidence limits (CL) 1.1 to 1.3 ° C/hr) than with normocapnia (0.7 ° C/hr, 95% CL 0.6 to 0.8 ° C/hr). In the hypercapnic study fraction of inspired carbon dioxide increased from 1.4 ±1.0 % to 7.0 ±1.4 %, minute ventilation increased from 15 ±7 l/min to 40 ±12 l/min, and oxygen saturation decreased from 97 ±1 % to 90 ±6 % (P<0.01). During the normocapnic study these parameters remained unchanged. In this study T_re cooling rate during snow burial was less than previously reported and was increased by hypercapnia. This may have important implications for pre-hospital treatment of avalanche burial victims.