The doubly labeled water method for measuring energy expenditure can be very sensitive to small differences in the ratio of the 2H to 18O isotope dilution spaces. Recently it has been suggested that the average ratio is higher than the 1.03 we previously recommended. We therefore combined the data from 99 recently studied subjects. Subjects (85 females and 14 males) were between the ages of 4 and 78 yr (mean = 34 yr) and between 10 and 52% (mean = 35%) fat. The average 2H-to-18O dilution space ratio was 1.034 +/- 0.014, which was very similar to the original assumption. As in recent reports, we did find that most of the variance (60%) was due to random analytic error and that there was no correlation between the dilution space ratio and age or body fat. However, in contrast to recent reports we found no evidence of a gender difference. Use of the constant dilution space ratio of 1.034 to recalculate CO2 product in published validation studies demonstrated improved accuracy, and thus the value of 1.034 is suggested for use in future studies.