There is conflicting evidence regarding a possible causal role for Chlamydia trachomatis in the development of preterm premature rupture of the membranes. We investigated the relative prevalence of endocervical infection with C. trachomatis and group B streptococci in patients with preterm premature rupture of membranes compared with a control group taken from the same obstetric population. C. trachomatis was isolated from 23/52 (44%) patients with preterm premature rupture of membranes versus 13/84 (15%) women in the control group (p less than 0.001). This association was independent of infection with group B streptococci or Neisseria gonorrhoeae. Group B streptococci were isolated from 16% of the patients with preterm premature rupture of membranes versus 4% of the control population (p less than 0.05). The risk of preterm premature rupture of membranes associated with group B streptococcal infection was independent of infection with C. trachomatis and N. gonorrhoeae. Endocervical infection with C. trachomatis did not significantly affect early maternal complication rates after delivery.