Objective To

determine the operational effectiveness of the South African programme for preventing mother-to-child transmission (PMTCT) of HIV in reducing rates of early transmission of infection. Methods Participants were mother–infant pairs who participated in the South African PMTCT programme between October 2002 and November 2004. This was a prospective cohort study. Three sites in different provinces were selected to represent differences in socioeconomic status and HIV prevalence. Data on antenatal care and labour ward care were obtained from maternal interviews and from reviews of medical records. A total of 665 mother–infant pairs in which the mother was HIV-positive were recruited and 588 (88.4%) were followed up at 3 or 4 weeks postpartum to determine the HIV status and vital status of the infant. Findings Rural participants were significantly poorer and their health care was significantly worse. Women of higher socioeconomic status and those who received better counselling were more likely to be treated with nevirapine. Rates of early HIV transmission ranged from 8.6 % to 13.7%. Maternal viral load was the only statistically significant risk factor for transmission. After adjusting for maternal viral load and prevalence of low birth weight, the odds of transmission were 1.8 times higher at the rural site. Controlling for having had> 4 antenatal visits and any delivery complication reduced the odds of transmission to 1.5 higher at the rural site. Conclusion Rates of early transmission of HIV in an operational setting using single-dose nevirapine administered both to mother and child are similar to those obtained in clinical trials. Scaling up access to antiretroviral regimens for women will further reduce transmission to infants. Bulletin of the World Health Organization 2007;85:466–473. Une traduction en français de ce résumé figure à la fin de l’article. Al final del artículo se facilita una traducción al español