OBJECTIVE: The authors describe the effect of ambient temperature on metabolic rate after thermal injury. SUMMARY BACKGROUND DATA: Thermal injury induces a hypermetabolic state, which is reported to increase with the extent of burn. The magnitude of this response is further influenced by ambient temperature. METHODS: The resting energy expenditure was measured by indirect calorimetry at ambient temperatures of 22, 28, 32, and 35 C. It was indexed to a calculated basal metabolic rate in normal volunteers and adult patients with burns involving at least 20% of the total body surface area who had no evidence of systemic infection. These measurements were performed between postburn days 6 and 21. RESULTS: The effect of ambient temperature on metabolic rate was measured in 44 burn patients and 8 normal volunteers. Burn size ranged from 20 to 97% total body surface area with a mean of 44 +/- 18.5% total body surface area. Metabolic rate did not change significantly in control subjects as ambient temperature was varied (p<0.05). Regression analysis showed that burn size and ambient temperature were significant determinants (p<0.01) of metabolic rate in the patients and that together these factors accounted for 55% of the variation observed (df adj. r(2)=0.55) across the range of ambient temperatures studied. Metabolic rate was independent of burn size at ambient temperatures of 32 and 35 C (p<0.02) and increased by a factor of 1.5 X basal metabolic rate. A further increase in metabolic rate, which was positively correlated with burn size, resulted from nonshivering thermogenesis at ambient temperatures 28 and 22 C. The magnitude of this response was greatest at 22 C. CONCLUSIONS: These findings suggest that the hypermetabolic response to thermal injury is maximal in burns as small as 20% total body surface area and that an additional burn size-dependent increase in metabolic rate results from heat loss at ambient temperatures below thermoneutrality.