Objective. Constitutional thinness (CT) is characterized by a low and stable BMI without any hormonal abnormality. In order to understand their weight steadiness, energetic metabolism was evaluated. Design. 7 CT, 7 controls and 6 anorexia nervosa (AN) young women were compared. CT and AN had a BMI < 16.5 kg/m2. Four criteria were evaluated: 1) energy balance including diet record, resting metabolic rate (RMR) (indirect calorimetry), total energy expenditure (TEE) (doubly labeled water), physical activity; 2) body composition (DXA); 3) biological markers (leptin, IGF-I, freeT3); 4) psychological profile of eating behavior. Results. The normality of freeT3 (3.7±0.5 pmol/L), IGF-I (225±93 ng/mL) and leptin (8.3±3.4 ng/mL) confirmed the absence of undernutrition in CT. Their psychological profiles revealed a weight gain desire. TEE (kJ/d) in CT (8382±988) was not found significantly different from that of controls (8793±845) and AN (8001±2152). CT food intake (7565±908 kJ/d) was found similar to that of controls (7961±1452 kJ/d) and higher than in AN (4894±703 kJ/d), thus explaining the energy metabolism balance. Fat-free mass (FFM) (kg) was similar in CT and AN (32.5±2.9 vs. 34.1±1.9) and higher in controls (37.8±1.6). While RMR absolute values (kJ/d) were lower in CT (4839±473) than in controls (5576±209), RMR values adjusted for FFM were the highest in CT. TEE/FFM ratio was also higher in CT than in controls. Conclusion. Energetic metabolism balance maintains a stable low weight in CT. An increased energy expenditure/free-fat mass ratio differentiates CT from controls and could account for the resistance to weight gain observed in CT.