The estimation of total daily energy expenditure (TEE) or the energy needs of athletes is important for maintaining a proper energy balance during training. This study was conducted to measure the TEE of Japanese middle- and long-distance runners using a heart-rate monitoring method (the HR method), and to ascertain the impact of three key energy expenditure (EE) components occurring within the day: sleep, exercise (training) and inactivity (other daily habitual activities). Additionally, energy balance was evaluated by comparing TEE with energy intake (EI). The mean TEE of six male athletes aged 19-21 y was 4,514+/-739 kcal/d and their mean EI was 3,784+/-91 kcal/d during the training period of 11 d. This negative energy balance led to a significant decline in body weight (p<0.01), indicating that proper care should be taken to balance energy during training. The mean daily EEs during sleep, inactivity, and exercise were 536+/-46 (12.0% of TEE), 1,722+/-145 (38.7%), and 2,255+/-658 (49.3%) kcal/d, respectively. There was a significant correlation between TEE and exercise EE (p<0.01) in the athletes. The TEE of the endurance athletes was higher than expected and significantly affected by exercise EE, indicating that the TEE of athletes, in general, must be carefully evaluated before incorporating nutritional support for them.