The relative proportions of prematurity (less than or equal to 2,500 gm less than 37 weeks' gestation) and intrauterine growth retardation-low birth weight (IUGR-LBW) (less than or equal to 2,500 gm, greater than or equal 37 weeks' gestation) among total LBW infants were studied in 11 regions in the developed world and 25 developing areas where known gestational ages and birth weights were reported. In developing countries a straight correlation was observed between total LBW incidence and IUGR-LBW incidence rates (r = 0.95; b = 0.98; p less than 0.001); in contrast, prematurity was not significantly associated with total LBW incidence. Data from the developed population showed results exactly opposite to those described for developing areas. Therefore, when the incidence of LBW is higher than 10%, it is almost exclusively due to the increase in IUGR-LBW infants, while prematurity remains almost unchanged (5% to 7%). When LBW incidence is less than 10% (mean=6%), preterm infants represent the major component of LBW. Environmental factors susceptible to changing socioeconomic conditions may be responsible for the observed differences.