1. Thirteen adult females and two males were overfed a total of 79-1 59 MJ (1900G38 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 104362 MJ (2500&87000 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 < 0.01) and between LBM gain and excess energy (r 0.49, P < 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.The relation of energy intake to body-weight status is a topic of current interest. When human subjects are studied under controlled conditions energy deficits lead to weight loss, and the greater the deficit the greater is the loss. Does the converse hold true for excess energy intake, and do such gains involve fat or lean, or both? Do women gain weight more easily than men? Do thin people gain weight less easily than those who are overweight?A number of short-term overfeeding studies have been done on normal human subjects during the past 50 years, ranging from the early studies of Cuthbertson et al. (1937) to the most recent one of Dallosso & James (1984a, b), and these have shown that nitrogen retention occurs as weight is gahed. However, longer-term studies (of 2 weeks or more) provide for better quantification of the energy cost of weight gain and of the induced changes in body composition. Several studies have been reported in which subjects were fed under controlled conditions and observations were made on changes in body composition for periods of 2-7 weeks (Wiley & Newburgh, 1931; Passmore et al. 1955a, b ; Miller & Mumford, 1967a, b ;Goldman et al. 1975;Norgan & Durnin, 1980; Webb & Annis, 1983). Almost all the subjFcts were males.The present report describes our studies of deliberate overfeeding of human subjects and uses these findings, together with those reported by others, to examine the questions posed previously. Overall, these studies encompass a wide range of energy intakes and of duration of overfeeding, a variety of diets, a range of initial body-weights, and include both sexes.