Data were collected on 1660 infants with birth weights between 501 to 1500 g who were inborn or admitted at 24 hours of age or less to 1 of the 12 National Institute of Child Health and Human Development (NICHD) Neonatal Research Network centers between August 31, 1994 and August 9, 1995, survived >7 days (168 hours) and were free of major congenital anomalies. Infants were weighed daily for a minimum of 14 days or until birth weight was regained, whichever occurred later, and then weekly. Recumbent length was measured weekly with a Premie Length Board. Head circumference (HC) was measured weekly. Midarm circumference (MAC) was measured weekly. Length, HC, and MAC measurements were each performed twice.