Based on the fact that there is evidence of an association between low birth weight and neonatal and infant morbi-mortality, the World Health Organization considered birth weight as the main isolated factor associated with infant's survival. A study was undertaken to identify risk factors associated with low birth weight, using epidemiological and demographic variables present on the Birth Certificates, the source of data for the System of Information on Newborn Infants of the Brazilian Ministry of Health, for the purpose of motivating the use and the analysis of data currently generated in hospitals.The distribution, by birth weight, of 14,784 single hospital newborn infants from five cities of the State of S. Paulo, Brazil, in an observation period of six months (1992), was analysed. Statistical analysis was based on point estimates (average, median and standard deviation) measures of association and confidence intervals (95%) of the prevalence ratios.The highest proportion (10.4%) of low birth weight infants (weight less than 2,500 g) occurred in Itararé, the city with the highest infant mortality rate in the State, the overall proportion being 7.5%. Significant statistical associations between low birth weight and sex (female), gestational age (less than 37 weeks), mother's age (adolescent and 35 or more years of age) and parity were detected. The use of Birth Certificates in epidemiological and public health studies is highly recommended because of their validity, great importance and high coverage.