The association between infection and preterm labour was studied in 72 women in spontaneous preterm labour between 26 and 34 weeks gestation and in 26 control subjects having an elective caesarean section at the same gestational age. The genital microbial flora of each group was studied comprehensively and included mycoplasmas, chlamydiae, ureaplasmas and anaerobes. Subsequent neonatal infection and chorioamnionitis was also studied. Abnormal bacterial colonization, the presence of ureaplasmas, heavy growth of mycoplasmas and chorioamnionitis were all found significantly more often in the study group. This supports the premise that a significant proportion of idiopathic preterm labour is associated with infection and this may permit better prediction and prevention of preterm birth. The continued use of tocolytics should depend upon the identification of the presence or absence of infection. Infection appeared to be the result rather than the cause of ruptured membranes. A recommendation with respect to the classification of abnormal or normal bacterial colonization between 26 and 34 weeks is suggested on the basis of strict criteria.