The obstetric risk factors for perinatal transmission of HIV-1 include preterm birth, prolonged rupture of the chorioamniotic membranes, and clinical and histological bacterial chorioamnionitis. A chronic chorioamnionitis precedes many cases of preterm labour and spontaneous rupture of membranes, whereas an acute chorioamnionitis is more common after rupture of the membranes at term. Amniotic fluid cytokines are raised in the presence of term and preterm intrauterine bacterial infections, and various cytokines seem able to attract HIV-1-infected leucocytes into the amniotic cavity and to increase replication of HIV-1. We postulate that the association of preterm birth and prolonged rupture of membranes with perinatal transmission of HIV-1 may result from an associated chronic or acute bacterial chorioamnionitis marked by the migration of HIV-1-infected maternal leucocytes into the amniotic cavity. Antibiotic treatment could prevent this sequence of events.