Metabolic efficiency was assessed in ovulatory eumenorrheic female distance runners and untrained control subjects of similar age, body weight, and fat-free mass (FFM). Energy intake (EI) was estimated from 3-d dietary records. Energy expenditure (EE) was determined during the same 3-d period from individual heart rate oxygen uptake (HR/VO2) curves during rest and exercise, 24-h HR records, and the thermic effect of meals. The runners and control subjects did not differ in resting metabolic rate statistically adjusted for FFM (kJ/min), the thermic effect of a test meal (kJ/3 h), the energy cost of submaximal physical activity, or EI. EE was higher (P = 0.01) in the runners. Reported EI was lower than EE in both the runners (P = 0.007) and control subjects, (P = 0.006), resulting in energy deficits of -4131 +/- 1185 kJ/d and -1652 +/- 456 kJ/d, respectively. These female runners did not exhibit an enhanced metabolic efficiency compared with the control subjects. It is possible that the energy deficit for both the runners and control subjects was due to both restricted eating and underreporting during the measurement period. Additional studies using longer measurement periods, more sophisticated technology (ie, doubly labeled water, more subjects, and subjects of varying menstrual and energy intake status) are needed to truly answer this question.