In order to study the usefulness of upper mid-arm circumference (MAC) and mid-arm circumference:head circumference ratio (MAC:HC) measurements in assessing longitudinal growth in hospitalized preterm infants, we prospectively measured weights, lengths, occipitofrontal head circumferences (OFC), MACs, MAC:HCs, weight/length for age, nutritional intakes, and serum transthyretin and albumin levels in 50 preterm, low-birth-weight, appropriate for gestational age newborn infants during their first 4 postnatal weeks and at hospital discharge. At some time during hospitalization, weight measurements were abnormal (greater than or equal to 2SD from the gestational age mean) in 48% of the infants as compared with 24% with abnormal MAC measurements (p = 0.002). Abnormal MAC:HCs occurred in 25% of the infants as compared with 68% with abnormal weight/length for age values (p less than 0.001). During weeks 2-4, when nutritional intakes were adequate and serum transthyretin and albumin levels were normal, mean weight gain velocity was less than intrauterine rates and was significantly slower than MAC velocities, which were at or greater than intrauterine rates (p less than 0.001). At discharge, when all infants were gaining weight at intrauterine rates, weight measurements were still abnormal in 28% of the infants as compared with 10% of infants who had abnormal MACs (p = 0.005). Similarly, only 12% of infants had abnormal MAC:HCs as compared with 25% of infants with abnormal weight/length for age values at discharge (p = 0.05). The MAC and MAC:HC are useful for assessing longitudinal growth in preterm infants since they do not overestimate the prevalence of malnourishment during periods of apparent protein-calorie sufficiency.