Amniotic fluid infection and its principal sequel bacterial infection of the newborn are major problems in obstetric pathology. The authors hav analysed prospectively 346 obstetrical casenotes over a period of 18 months in order to try to describe and specify the clinical features and to try and work out using bacteriological tests the risk of infecting the neonate by materno-fetal transmission. They compared a control group (27 case histories) and the group at risk (235 cases). 4% of the control group and 18.7% of the "at risk" group were biologically infected (p less than 10(-2)). The following are among the classical clinical criteria to correlate this risk: maternal pyrexia, premature delivery, urinary tract infections and fetal distress; an apparent lessening in active fetal movements is also well correlated to the risk of infection. The bacteriological criteria are: the presence of quantities of altered polymorphonuclear cells in the amniotic fluid--this seems to be more important than others (sensitivity = 70%, specificity = 89%, but positive predictive value = 60% and negative predictive value = 93%). From this study it should be possible to work out antenatally whether the newborn baby will be at high risk of developing an infection in order to consider giving antibiotics to prevent materno-fetal transmission.