The doubly labeled water method for the measurement of average daily metabolic rate (ADMR), combined with a measurement of resting metabolic rate, permits the calculation of energy expenditure for physical activity under normal daily living conditions. This procedure was used to evaluate the use of movement registration for physical activity assessment under daily living conditions. Subjects were 16 men and 14 women with normal weight (body mass index (BMI) 24.6 +/- 2.4 kg/m2). Their body movement was registered with a triaxial accelerometer over a 7-day interval, simultaneous with an ADMR measurement with a doubly labeled water method. Resting metabolic rate was measured overnight in a respiration chamber (sleeping metabolic rate (SMR)) at the start of the ADMR measurement. Subjects did wear the accelerometer during waking hours. Accelerometer output (AO, counts/min) was related to physical activity as quantified by adjustment of ADMR for SMR. Additional studies were performed in 11 subjects with anorexia nervosa (BMI 16.7 +/- 1.7 kg/m2) and 8 subjects with morbid obesity (BMI 45.3 +/- 6.8 kg/m2). AO explained most of the variation in ADMR, after adjustment for SMR (R2 = 0.64, SEE = 0.9 MJ/d) Average AO was 1108 +/- 293, 1144 +/- 318, and 946 +/- 391 for subjects with normal weight, anorexia nervosa, and morbid obesity, respectively, and was not significantly different between the three groups. However, in the anorectics AO was significantly related to body mass index (r = 0.84, (p < 0.01), subjects with a BMI17 kg/m2 were equally or more active compared with control subjects, while subjects with a BMI < 17 kg/m2 were equally or less active compared with control subjects. In the morbid obese group, 5 of the 8 subjects had a low activity level (AO < 900 counts/day) and the other 3 had a high activity level (AO1150 counts/day). The triaxial accelerometer is an objective method that can be used to quantify physical activity related energy expenditure and to distinguish differences in activity levels between individuals.