The metabolic equivalent (MET) is a widely used physiological concept that represents a simple procedure for expressing energy cost of physical activities as multiples of resting metabolic rate (RMR). The value equating 1 MET (3.5 ml O2·kg-1·min-1 or 1 kcal·kg-1·h-1) was first derived from the resting O2 consumption (V̇O2) of one person, a 70-kg, 40-yr-old man. Given the extensive use of MET levels to quantify physical activity level or work output, we investigated the adequacy of this scientific convention. Subjects consisted of 642 women and 127 men, 18-74 yr of age, 35-186 kg in weight, who were weight stable and healthy, albeit obese in some cases. RMR was measured by indirect calorimetry using a ventilated hood system, and the energy cost of walking on a treadmill at 5.6 km/h was measured in a subsample of 49 men and 49 women (26-45 kg/m2; 29-47 yr). Average V̇O 2 and energy cost corresponding with rest (2.6 ± 0.4 ml O 2·kg-1·min-1 and 0.84 ± 0.16 kcal·kg-1·h-1, respectively) were significantly lower than the commonly accepted 1-MET values of 3.5 ml O 2·kg-1·min-1 and 1 kcal·kg-1·h-1, respectively. Body composition (fat mass and fat-free mass) accounted for 62% of the variance in resting V̇O2 compared with age, which accounted for only 14%. For a large heterogeneous sample, the 1-MET value of 3.5 ml O2· kg-1·min-1 overestimates the actual resting V̇O2 value on average by 35%, and the 1-MET of 1 kcal/h overestimates resting energy expenditure by 20%. Using measured or predicted RMR (ml O2·kg-1·min-1 or kcal·kg-1·h-1) as a correction factor can appropriately adjust for individual differences when estimating the energy cost of moderate intensity walking (5.6 km/h). Copyright © 2005 the American Physiological Society.