Postnatal body weight changes were assessed in 385 surviving infants with birth weights of less than 2,500 g. Body weight was measured daily between birth and 45 days of age. Infants were grouped according to 100-g birth weight categories, and mean body weight changes for each group were compared. Initial postnatal weight loss occurred in each group and ranged between 7.9% and 14.6% of birth weight. Mean postnatal weight loss was greater in the lowest birth weight groups, but considerable variability was observed among individual infants. Duration of postnatal weight loss was similar among all birth weight groups. Weight gain usually began between four and six days of age, and the rate of weight gain expressed as grams per kilogram per day was similar in all birth weight groups.