In order to evaluate the influence of breast feeding upon selected nutritional parameters of lactating women, several anthropometric and biochemical measurements were performed during the interval between 40 and 180 postpartum days in 54 women in full and satisfactory nursing. Only one significant change was detected in measurements done for nutritional assessment: loss of body weight. No changes were detected in hemoglobin levels or in serum protein and albumin. There was high variability in body weight changes with a mean loss of 1.5 kg due especially to a loss in fat body mass and, in a lesser proportion, to loss in lean body mass. The mean food intake did not change during the observation period. The mean caloric intake was 2,771 cal/day at the beginning and 2,737 cal/day in the end of the study. A significant correlation was established between the individual energetic intake and the body weight changes of lactating women. The allowance of 2,750 cal/day suggested by FAO/WHO for lactating women seems adequate for the health of women in a normal nutritional condition, but apparently insufficient to overcome undernutrition in women who initiate lactation with this condition. In spite of the small nutritional impact observed, our results support the need for food supplementation programs destined to lactating women, with special orientation to undernourished women of low socioeconomic level.