Costa Rica has experienced a marked reduction in infant mortality--both in the neonatal and post-neonatal components--in the last decade. The decline could be related to improvements in the pattern of fetal growth. The present report analyses the birth weight of newborns from a random sample of all births in the Republic of Costa Rica during 1970 and 1975. While the average birth weight in that period was 3100 grams, the prevalence of low birth weight neonates decreased from 9 to 7 per cent in five years. The provinces of Limon and San Jose exhibited the highest frequency of low birth weight. Women 20 to 29 years old had babies with better or optimal body weight. Age, marital status and occupation of the mother appeared correlated with birth weight. A relationship between changes in fetal growth and changes in maternal, perinatal and neonatal mortality is apparent. The present situation of birth weight places Costa Rica among the countries in transition with a clearer perspective to attain an even higher infant survival in the near future. In this regard, several measures oriented toward prevention of low birth weight are recommended.