1. In order to evaluate factors influencing thermogenesis in obesity, energy expenditure was measured before and during an adrenaline infusion (25 ng min-1 kg-1 ideal body weight for 30 min) in 22 obese females. 2. Thermogenic responses were related to body morphology, age and biochemistry. In addition, thermogenic responses were related to cardiovascular responses by simultaneously measuring blood pressure, pulse rate and cardiac output using Doppler sonography. 3. Resting energy expenditure was predicted by body weight, lean body mass and fat mass. 4. Adrenaline-induced thermogenesis was predicted by fasting insulin, low basal respiratory quotient and body fat. 5. There was a significant relationship between the cardiac output and thermogenic responses to adrenaline (r = 0.63 P < 0.015) but there was no relationship to the heart rate or blood pressure responses. For every 1% increase in energy expenditure, there was a 5% increase in cardiac output. 6. In conclusion, the factors predicting resting energy expenditure and adrenaline-induced thermogenesis are different. Increased lipid oxidation and central fat distribution (with hyperinsulinaemia) are associated with a greater thermogenic response. The proportionately greater cardiac output responses may have implications for thermogenic agents designed to induce weight loss.