The energy requirements of pregnancy is a topic of considerable uncertainty because the apparent energy costs--to supply the energy required for the changes in maternal and fetal tissues and to cover the increase in basal metabolic rate (BMR), amounting to the equivalent of an extra 250-300 kcal/day--are rarely paralleled by actual findings. However, few studies have investigated this problem in a controlled longitudinal and statistically acceptable fashion. In this study, measurements of energy intake, BMR, body weight and fatness, physical activity, and mechanical efficiency of movement were made on 162 women in Scotland at 2- to 4-wk intervals throughout pregnancy, and data from 98 of these women were also obtained in the prepregnant state. Data from a parallel study in the Netherlands are also given. The gains in body weight, fatness, the fetal and placental weights, and the increase in BMR were all at the normal expected values. However, increases in energy intake were on the order of only 100 kcal/day during the second and third trimesters. Comparatively small reductions in physical activity, which would be difficult to measure precisely, are described that could account for the difference between actual energy intake and the energy costs of pregnancy.