An increase in preterm deliveries in Ribeirão Preto stimulated an analysis of possible explanatory factors. Two cohorts of singleton livebirths were studied, the first based on 6746 births in 1978-9 and the second based on 2846 births in 1994. A logistic regression was carried out to assess the association of preterm birth with several sociodemographic, behavioural and clinical variables, including year of survey. Delivery in private settings compared with a public setting, maternal age of < or = 17 compared with any other age group, and mothers who had had previous abortions and previous stillbirths were associated with greater rates of preterm birth. Although there was an increase in preterm birth rates regardless of mode of delivery, the increase was greater in the caesarean section group than in the vaginal delivery group. Over the study period, deliveries in private hospitals and caesarean section operations increased markedly (from 4% to 36% and from 30% to 51% respectively). Caesarean section may be the main contributor to the increase of preterm birth rate in this study. It is essential to ensure that health-care staff, especially those in private facilities, are properly educated and audited.