In a prospective study of colonization with group B streptococci (GBS) among 6,706 parturients, we found statistically increased incidences of premature rupture of membranes (PROM) and preterm delivery at less than 32 weeks' gestation among women colonized with GBS. PROM occurred in 8.1% of the total population but in 15.3% of the colonized population (p less than 0.005). Preterm delivery at less than 32 weeks' gestation occurred in 1.8% of the total population but among 5.4% of women colonized with GBS (p less than 0.005). The data suggest a causal relationship between GBS colonization and events leading to preterm birth. The possible impact of eradication of colonization with GBS on prematurity is considered.