The effect of overfeeding on energy expenditure was investigated in 23 young men subjected to a 353-MJ energy intake surplus over 100 d. The major part of this excess (222 MJ) was stored as body energy. The increase in energy cost of weight maintenance amounted to 52 MJ and was proportional to body weight gain. When it was added to the obligatory cost of fat and fat-free mass gains, the overall increase in energy expenditure amounted to a mean of 100 MJ. Four months after overfeeding, subjects had lost 82%, 74%, and 100% of the overfeeding gain in body weight, fat mass, and fat-free mass, respectively. We conclude that 1) in response to overfeeding, two-thirds of the excess energy intake is stored as body energy; 2) overfeeding induces an increase in energy cost of weight maintenance proportional to body weight gain, and 3) preoverfeeding energy balance tends to be restored when nonobese individuals return to their normal daily-life habits.