In affluent populations, breast-feeding women often wish to return to their prepregnancy weight as soon as possible postpartum and may restrict energy intake or increase exercise to achieve this goal. Gradual weight loss (< or = 2 kg/mo) seems to have no adverse effect on milk volume or composition, provided that the mother is not undernourished and is breast-feeding her infant on demand. Aerobic exercise improves cardiovascular fitness and does not affect milk energy transfer to the infant, but exercise alone is not likely to increase the rate of weight loss unless dietary intake is controlled. Less information is available on the effect of rapid weight loss. Data from a recent randomized intervention trial indicate that a short-term (11 d) energy deficit of 35%, achieved by dieting or a combination of dieting and increased exercise, results in weight loss > 1 kg/wk and does not adversely affect lactation. Exercise enhances maintenance of lean body mass and is therefore a recommended component of any weight loss program. Maternal plasma prolactin concentration generally increases under conditions of negative energy balance, which may serve to protect lactation. Further research is required on the longer-term effect of energy restriction and on the effects of energy balance in lactating women with low fat reserves.