A prospective study was conducted to evaluate the usefulness of amniocentesis in patients with premature rupture of membranes (PROM) for the prediction of occult or impending intrauterine infection and the assessment of fetal maturity. Fifty-nine patients with PROM between 28 and 35 weeks' gestation, without apparent infection, were evaluated sonographically for possible amniocentesis. Thirty patients had successful amniocenteses for a success rate of 51%. Fifteen patients had a mature lecithin/sphingomyelin (L/S) ratio and were delivered immediately without respiratory distress syndrome (RDS) or other neonatal complications. All amniotic fluids were evaluated by Gram stains and cultures. Nine of the 30 patients had positive cultures, with a high incidence of subsequent development of am-nionitis and other infectious morbidity. In the 21 patients with negative cultures, there were 19 without any infectious morbidity. Amniocentesis appears in this study to be a useful method for selecting the patients who have mature fetuses and/or who are more likely to develop amnionitis.