OBJECTIVE: Our aim was to study the response to antiretroviral treatment among women exposed to single-dose nevirapine (NVP) and/or short-course zidovudine (ZDV; with or without lamivudine [3TC]) for the prevention of mother-to-child transmission of human immunodeficiency virus (HIV) infection. METHODS: All HIV type 1-infected women who initiated antiretroviral treatment with stavudine or ZDV, 3TC, and NVP or efavirenz were eligible for the MTCT-Plus program in Abidjan, Ivory Coast. Exposed women had received either single-dose NVP alone or short-course ZDV (with or without 3TC) plus single-dose NVP during previous pregnancy. Genotypic resistance testing was performed at week 4 after delivery. Virologic failure was defined as a plasma HIV RNA level >500 copies/mL 12 months after initiation of antiretroviral treatment. RESULTS: Among 247 women who received antiretroviral treatment, 109 (44%) were unexposed; 81 had received short-course ZDV with 3TC, as well as single-dose NVP; 5 had received short-course ZDV plus 3TC; 50 had received short-course ZDV plus single-dose NVP; and 2 had received single-dose NVP alone. No ZDV mutation was detected in the 115 women whose specimens were available for genotypic testing; 11 (15.1%) of 73 women with 3TC exposure who were tested after delivery had 3TC resistance mutations. Three (4.3%) of 69 women exposed to short-course ZDV and 3TC plus single-dose NVP and 16 (38.1%) of 42 women exposed to short-course ZDV plus single-dose NVP had NVP resistance mutations. Antiretroviral treatment was initiated a median of 21 months after the intervention to prevent mother-to-child HIV transmission (median CD4(+) T lymphocyte count, 188 cells/mm(3)). Month 12 virologic failure was identified in 42 (19.2%) of 219 women for whom data were available, and multivariate analysis revealed that it was associated with poor adherence to treatment (adjusted odds ratio [aOR], 12.7; 95% confidence interval [CI], 3.0-53.9), postpartum 3TC resistance mutations (aOR, 6.9; 95% CI, 1.1-42.9), and a baseline CD4(+) T lymphocyte count <200 cells/mm(3) (aOR, 0.3; 95% CI, 0.2-0.8). NVP resistance was not associated with virological failure (aOR, 1.8; 95% CI, 0.5-6.5). CONCLUSIONS: Our study found that poor adherence and 3TC resistance acquired after the intervention to prevent mother-to-child transmission of HIV infection were associated with virologic failure in women who initiated antiretroviral treatment.