We investigated the contributions of low energy expenditure and high energy intake to excessive weight gain in infants born to overweight mothers. The subjects were infants of 6 lean and 12 overweight mothers, recruited soon after birth. Total energy expenditure and metabolizable energy intake were measured with a new doubly labeled water method over a period of seven days when the infants were 3 months of age, and the postprandial metabolic rate was measured by indirect calorimetry when the infants were 0.1 and 3 months of age. The results were related to weight gain in the first year of life. No significant difference was observed between infants who became overweight by the age of one year (50 percent of infants born to overweight mothers) and those who did not, with respect to weight, length, skinfold thicknesses, metabolic rate at 0.1 and 3 months of age, and metabolizable energy intake at 3 months. However, total energy expenditure at three months of age was 20.7 percent lower in the infants who became overweight than in the other infants (means +/- SE, 256 +/- 27 and 323 +/- 12 kJ per kilogram of body weight per day; P less than 0.05). This difference could account for the mean difference in weight gain. These data suggest that reduced energy expenditure, particularly on physical activity, was an important factor in the rapid weight gain during the first year of life in infants born to overweight mothers.