1. Thirteen adult females and two males were overfed a total of 79-159 MJ (19,000-38,000 kcal) during a 3-week period at the Clinical Research Center, Rochester. The average energy cost of the weight gain was 28 kJ (6.7 kcal)/g, and about half the gain consisted of lean body mass (LBM) as estimated by 40K counting. 2. A survey of the literature disclosed twenty-eight normal males and five females who had been overfed a total of 104-362 MJ (25,000-87,000 kcal) under controlled conditions: twenty-five of these had assays of body composition, and three had complete nitrogen balances. 3. When these values were combined with those from our subjects (total forty-eight), there was a significant correlation between weight gain and total excess energy consumed (r 0.77, P less than 0.01) and between LBM gain and excess energy (r 0.49, P less than 0.01). Based on means the energy cost was 33.7 kJ (8.05 kcal)/g gain and 43.6% of the gain was LBM; from regression analysis these values were 33.7 kJ (8.05 kcal)/g gain and 38.4% of gain as LBM. 4. Individual variations in the response could not be explained on the basis of sex, initial body-weight or fat content, duration of overfeeding, type of food eaten, amount of daily food consumption or, in a subset of subjects, on smoking behaviour. 5. The average energy cost of the weight gain was close to the theoretical value of 33.8 kJ (8.08 kcal)/g derived from the composition of the tissue gained.