Recent publications have highlighted the controversy regarding the significance of Lancefield Group B Streptococcal (GBS) colonization in pregnancy and preterm delivery. In this prospective study vaginal swabs from 692 women at approximately 24 weeks' gestation were cultured for GBS. GBS was detected in 91 (13.2%) women. The rate of preterm labour (PTL) (less than 37 weeks) was significantly higher in GBS positive women than in GBS negative women (18.7% versus 5.5%; p less than 0.001). This association remained significant even when patients with other recognized factors predisposing to PTL were excluded (11.5% versus 3.9%; p less than 0.001). The rate of premature rupture of membranes (PROM) was also significantly higher in GBS positive women (9.9% versus 2.7%; p less than 0.005) and remained significantly higher when patients with other recognized risk factors were excluded (6.1% versus 1.8%; p less than 0.025). These results unequivocably show that pregnant women who are vaginal carriers of GBS have a significantly increased risk of PROM and PTL.