Summary. Two studies were conducted during 1989-90 in Central Sudan to determine the incidence of low birthweight and to ascertain the major risk factors which influence birthweight. In a hospital-based investigation, surveillance of all births was accompanied by a nested case-control study, and in a community based investigation, all midwife-assisted births were studied. There were 4868 and 1523 livebirths among the hospital and community populations, respectively. The incidence of low birthweight was 18.1% in the community and 8.2% in the hospitals. The ratio of term to pre-term low birthweight was 2.9 in the community but only 1.3 in the hospitals. Several risk factors showed consistent and significant associations with low birthweight in the hospital and community studies. Two important and modifiable predictors of term and preterm low birthweight were low maternal weight and malaria infection during pregnancy. Other risk factors included low socio-economic status and, among the hospital population, lack of antenatal care, short birth intervals, poor obstetric history and complications of pregnancy.