Adaptive thermogenesis is the dissipation of energy as heat. Earlier studies, on retrospective data, indicate that brown adipose tissue (BAT) activity is greater in women than in men. We aimed to characterize BAT thermogenesis using infrared thermography (IRT) in healthy men (n=14; age 23.07 ± 0.7 years, BMI 23.22 ± 0.7 kg/m2) and in women during 2 stages of the menstrual cycle (luteal, n=9; age 25.22 ± 1.7 years, BMI 21.56 ± 0.4 kg/m2 and follicular, n=11; age 24.64 ± 1.2 years, BMI 22.12 ± 0.9 kg/m2). IRT measured cutaneous temperature at the supraclavicular region (BAT positive) and the manubrium as control (BAT negative). Temperature recordings were at 1 min intervals, at an ambient temperature of 18.5-20°C. To activate cold-induced thermogenesis, one hand was immersed into water at 15°C for 5 min. Post-prandial thermogenesis was stimulated by the consumption of a liquid meal (Ensure, 10 kcal/ kg body weight). Females had greater (P<0.05) thermogenic responses to cold and dietary stimuli than males; this effect was greater (P<0.05) during the luteal phase of the menstrual cycle. With cold-exposure, the increase in BAT temperature was lower (P<0.05) in females during the follicular phase than the luteal phase. Similarly, there was a trend towards reduced meal-induced thermogenesis in women during the follicular phase. Regression analyses demonstrated correlations between the degree of temperature change after a meal and serum progesterone levels (P<0.01, R2=0.12), and temperature response to cold and estrogen levels (P<0.05, R2=0.13). There was an inverse relationship between BAT temperature response and serum cortisol (P<0.01, R2=0.17) and testosterone (P<0.05, R2=0.13) concentration. In summary, females typically display greater thermogenic responses than males, but there are different responses in females in the luteal and follicular phases of the menstrual cycle. Differences in thermogenic responses are associated with variations in circulating sex and stress steroid concentrations.