Computerized recording of dynamic skinfold thickness measurements was performed on low birthweight infants. Four skinfold sites were measured daily for the first 7 days postnatally and then at intervals up to 4 weeks of age. The percentage of fall of the skinfold between 0.4 and 30 seconds after caliper application was shown to correlate best with postnatal weight loss until minimum weight was reached. Of the 24 infants studied in detail, only two had a correlation coefficient less than 0.75 between changes in the percentage of fall of the skinfold and postnatal weight changes. There was, however, marked individual variation in the amount of weight loss for a given reduction in the percent of the skinfold. Assuming that most of the postnatal weight loss of low birthweight infants is due to reduction of extracellular volume, dynamic skinfold thickness appears to be a good noninvasive measure thereof.