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To evaluate the effect of maternal, obstetric, neonatal and post-natal factors on the risk of vertical transmission of HIV-1.Multicentre retrospective cohort study.Obstetric and paediatric clinics in four cities in Sao Paulo State, Brazil.Child's HIV-1 infection status.Data were collected by standardized record abstraction and interview on 553 children born to women identified as HIV-1-infected before or at delivery. Paediatric infection was determined by immunoglobulin G anti-HIV-1 tests at age 18 months or by AIDS diagnosis at any age. Multivariate logistic regression was used to assess the effect of potential risk factors on vertical transmission of HIV-1.HIV-1 infection status was determined for 434 children (follow-up rate of 78%); 69 were classified as HIV-1-infected [transmission risk, 16%; 95% confidence interval (CI), 13–20%]. In multivariate analysis, advanced maternal HIV-1 disease [odds ratio (OR), 4.5; 95% CI, 2.1-9.5], ever breastfed (OR, 2.2; 95% CI, 1.2–4.2), child's negative Rhesus blood group (OR, 2.5; 95% CI, 1.2–5.5), third trimester amniocentesis (OR, 4.1; 95% CI, 1.2–13.5) and black racial group (OR, 0.3; 95% CI, 0.1–0.9) were independently and significantly associated with mother-to-child transmission of HIV-1. Transmission was increased marginally with prematurity, more than 10 lifetime sexual partners and prolonged duration of membrane rupture. No association was found between child's HIV-1 infection and mode of delivery or serological evidence of syphilis during pregnancy.These findings support the importance of severity of maternal HIV-1 disease in the risk of vertical transmission of HIV-1, indicate measures to reduce transmission by avoiding amniocentesis and breastfeeding and suggest that race and Rhesus blood type may be markers for genetic susceptibility to infection.