Causes of premature rupture of the fetal membranes were explored in a study of 10,460 pregnancies. Five risk factors were analyzed: Cigarette smoking, coitus, parity, prior surgery of the cervix, and chorioamnionitis. To minimize the effects of other influences the author compared the rates of premature rupture in successive pregnancies of the same mothers when the individual risk factors being analyzed were present in only 1 of the 2 pregnancies. Preterm delivery due to premature rupture of the membranes was 11 times more frequent when recent coitus was present in conjunction with chorioamnionitis than when both were absent. Such ruptures were increased only 3-fold when either recent coitus or chorioamnionitis was present without the other.