A case-control study was conducted in Natal, north-east Brazil to determine the risk factors for low birthweight (LBW). Cases were 429 preterm and 422 intrauterine growth retarded (IUGR) singleton infants. Controls were 2555 infants of normal birthweight and gestational age. The prevalence of LBW was 10% (5.1% preterm and 4.9% IUGR). Logistic regression was used to estimate the adjusted odds ratios of LBW, and attributable risk per cent (AR%) was used to estimate the proportion of LBW that might be prevented. The preventable determinants of preterm delivery were births to women less than 20, (AR = 7.1%), low maternal weight less than 50 kg (AR = 20.5%), smoking during pregnancy (AR = 14.6%) and infrequent antenatal visits (AR = 28.1%). Other important determinants of preterm delivery were prior LBW births, gestational illness and vaginal bleeding. The main preventable causes of IUGR were low maternal weight (AR = 17.8%), low maternal education (AR = 11.6%), smoking (AR = 14.8%), and inadequate antenatal care (AR = 11.6%). Other risk factors for IUGR include primiparity, prior LBW births, and illness during gestation. In this population, the focus of short-term preventive programme should be improvement in maternal nutrition, cessation of smoking, reduction of births to women under 20, and improved antenatal care.