The authors report a prospective study of correlation between histopathological alterations of the placenta, risk factors and early-onset bacterial infections in 224 premature newborns. They used a mathematical model for evaluation and prediction of neonatal bacterial infection according to the localization in chorioamniotic tissues (chorioamniotic plate, amniotic membranes and umbilical cord) invaded by leukocytes. Septicemia, pneumonia or omphalitis were documented in 45 (20%) infected premature newborns and inflammatory lesions in the placenta were observed in all of them. In order of statistical significance, the most important variables for early-onset bacterial neonatal infection were invasion of the chorioamniotic plate, amniotic membranes and umbilical cord tissues by PMNL (P < 0.0000), premature rupture of membranes (P < 0.0000), birthweight lower than 1500 g (P < 0.0000), gestational age under 34 weeks (P < 0.0001), foul smell (P < 0.0038), no antibiotics before delivery (P < 0.0066) and intrapartum fever (P < 0.0087). By logistic stepwise multiple regression analysis, invasion of fetal chorioamniotic plate and of amniotic membranes by leukocytes were the only statistically significant variables. The probability of neonatal infection in premature newborns, when polymorphonuclear neutrophils were present in chorioamniotic plate and in amniotic membranes, was 62.5%, while the probability was 0.5% when these tissues were normal. These data suggest that histological chorioamnionitis has to be considered as an important risk factor for early-onset infection in premature newborns.