Clinical reports consistently comment on high physical activity for anorexia nervosa patients but provide few quantitative measurements. To assess activity, total daily energy expenditure (TDEE) by doubly labeled water, basal metabolic rate (BMR), and thermic effect of meals (TEM) were measured in six female outpatients with anorexia nervosa (67% of ideal body weight) and age-, sex-, and height-matched to six control subjects. Anorexia nervosa patients expended more energy as physical activity than did control subjects [0.084 +/- 0.012 vs 0.044 +/- 0.008 MJ/kg body wt, respectively (20.1 +/- 3.0 vs 10.5 +/- 1.9 kcal/kg body wt, respectively), P less than 0.04], although they had a lower BMR [4.17 +/- 0.37 vs 5.52 +/- 0.15 MJ/d, respectively (997 +/- 89 vs 1319 +/- 37 kcal/d, respectively), P less than 0.01]. TDEE and TEM were similar in both groups. There was a reduction in serum triiodothyronine (T3; 1.20 +/- 0.15 vs 2.04 +/- 0.13 nmol/L, respectively; P less than 0.003) and a slight reduction in serum thyroxine (T4); reverse T3, thyrotropin, free T4, serum cortisol, and adrenocorticotropin values were normal. BMR correlated with total body weight and fat-free mass. These results provide quantitative evidence for increased physical activity in anorexia nervosa despite profound underweight and hypometabolism.