OBJECTIVES: In Africa, single-dose nevirapine (NVPsd), short regimens of zidovudine (ZDV) or ZDV + lamivudine (3TC) are recommended to prevent peripartum mother-to-child HIV transmission (PMTCT). We evaluated the 6-week field efficacy of two more PMTCT drug combinations. DESIGN: An open-label intervention cohort in Abidjan. METHODS: In 2001-2002, consenting women started oral ZDV 300 mg twice daily (bid) at > or =36 weeks of gestation, with 600 mg of ZDV + 200 mg NVPsd orally at beginning of labour. In 2002-2003, the antepartum regimen at > or =32 weeks comprised ZDV as previously + 3TC 150 mg bid; the labour dose comprised ZDV + NVPsd as previously + 300 mg 3TC orally. Neonates received ZDV syrup (2 mg/kg per 6 h) for 7 days + NVPsd syrup (2 mg/kg) on day 2 in both periods. Each woman was assisted to either use breast milk substitutes or breastfeed exclusively. Paediatric HIV infection was diagnosed by plasma HIV RNA viral load at 4 weeks, confirmed at 6 weeks. The reference group was a cohort receiving a short regimen of ZDV > or = 36-38 weeks in 1995-2000 in the same population. RESULTS: A total of 1144 HIV-infected pregnant women were included: 351 with ZDV, 420 with ZDV + NVPsd and 373 with ZDV + 3TC + NVPsd; 1010 livebirths were eligible for analysis; 79 children were HIV-infected peripartum. Six-week transmission probability was 6.5% [95% confidence interval (CI), 3.9-9.1%) with ZDV + NVPsd, a 72% reduction compared with ZDV alone (95% CI, 52-88%; P = 0.0002 adjusted on maternal CD4, clinical stage and breastfeeding). It was 4.7% (95% CI, 2.4-7.0%) with ZDV + 3TC + NVPsd (P = 0.34 compared with ZDV + NVPsd). CONCLUSIONS: A short-course of ZDV + NVPsd prevents most peripartum HIV transmission in Africa. This regimen could be added to international guidelines.