Malaria during pregnancy reduces birth weight, and low birth weight is a major determinant of infant mortality. The authors estimated the impact of malaria during pregnancy on infant mortality in a Karen population living in Thailand. Between 1993 and 1996, a cohort of 1,495 mothers and their infants was followed weekly from admission of the mother to antenatal clinics until the first birthday of the infant. Both falciparum malaria and vivax malaria during pregnancy were associated with low birth weight but did not shorten gestation. Febrile illness in the week before delivery was associated with premature birth. Preterm and full-term low birth weight and fever in the week before delivery were associated with neonatal mortality. Maternal fevers close to term were also associated with the deaths of infants aged between 1 and 3 months, whereas no risk factors could be identified for deaths that occurred later in infancy. Thus, malaria during pregnancy increased neonatal mortality by lowering birth weight, whereas fever in the week before birth had a further independent effect in addition to inducing premature birth. The prevention of malaria in pregnancy and, thus, of malaria-attributable low birth weight should increase the survival of young babies.