PIP: Data on all births and infant deaths in the US were derived from reproduced computer tapes provided by local and state vital statistics units to analyze the risk factor involving intrauterine growth retardation (IUGR) as manifested by low birth weight for gestational age. The findings address the mortality of these infants and their morbidity as determined by an assessment at 1 year of age compared with infants who were comparably small but of appropriate weight for gestational age. In each region, matching of birth and death certificates provided data to assess neonatal and postneonatal mortality rates in 1974 and 1975 by birth weight; in 6 of the 8 areas gestational age based on the date of the last menstrual period was also available. Morbidity data for infants surviving to 1 year of age were obtained through a household survey of a random sample of infants born in the 8 regions during a 6-month period in 1976 and identified through birth certificates. The samples were selected to include all of the infants weighing 2500 gm or less at birth and, on average, 3% of those weighing more than 2500 gm at birth for all regions combined. The survey instrument consisted of 2 parts: an interview with the mother to obtain sociodemographic data and information on the child's illnesses, hospitalizations, malformations, participation in special followup programs, usual source of care, and health care received; and observations on the child. The number of infants visited totaled 4989, including 3179 infants with birth weights of 2500 gm or less, 1777 infants weighing over 2500 gm at birth, and 33 infants of unknown birth weight. Low birth weight for gestational age (SGA) was defined statistically as the lowest 25th percentile of weight at each week of gestation. Over 2/3 of infants in each birth weight group under 3000 gm were categorized as SGA, even though SGA was defined as the lowest 25th percentile of infants at each week of gestation. The SGA infants were more likely to survive to 1 year of age, largely because of greater survival in the neonatal period, in all weight categories under 2501 gm. SGA survivors under 2001 gm were more likely than appropriate weight for gestational age (AGA) survivals to have severe, moderate, or mild impairment at 1 year, particularly at birth weights less than 1500 gm. Very small (1500 gm or less) SGA and AGA infants who survived and who did not have severe, moderate, or mild impairment appeared to be at approximately equal risk of rehospitalization and prolonged illnesses; SGA and AGA infants at birth weights above 2501 gm appeared to be at equal risk of problems manifest at 1 year of age. Table 1 provides data on only those rehospitalized children who did not have congenital defects or developmental delay. There were no consistent differences between AGA and SGA infants in the duration of original hospitalization, likelihood of rehospitalization, or total days of hospitalization in the 1st year of life.