OBJECTIVE: The aim of this study was to estimate the field efficacy of the first routine programme for the prevention of mother-to-child transmission (PMTCT) of human immunodeficiency virus (HIV) initiated in South Africa, in the subdistrict of Khayelitsha. METHODS: A consecutive sample of 658 mother-infant pairs, identified from the PMTCT register from 1 March to 30 November 2003, were identified for enrolment in this study. Details of the regimen received were established and HIV status of the infants at between 6 and 10 weeks of age was determined by qualitative DNA polymerase chain reaction. Zidovudine (AZT) was provided antenatally from week 34 of gestation and during labour. Infant formula milk was-offered to mothers who chose not to breastfeed. The protocol was amended in July 2003 such that women who had received < 2 weeks of treatment with AZT were given a single dose of nevirapine (NVP) at the onset of labour, and the infant received a weight-adjusted dose of NVP within 72 h of delivery. RESULTS: Of the 535 mother-infant pairs (81%) eventually included in the study, 410 (77%) received an effective PMTCT intervention according to the protocol. The rate of transmission of HIV from mother to child was 8.8% (95% confidence interval (CI), 6.2-10.9). A maternal age of > 25 years was the only significant independent risk factor for transmission (odds ratio, 2.12; 95% CI, 1.14-4.07). CONCLUSION: The results of this study demonstrate the feasibility and effectiveness of a large-scale PMTCT programme in an urban public-sector setting.