['To model the effect of health systems performance on rates of mother-to-child HIV transmission.', 'We modeled the effect of variation in performance of the multiple steps of different prevention of mother-to-child transmission (PMTCT) protocols using hypothetical and reported data.', 'Data from a PMTCT program in a large province in South Africa was used to compare model predictions with reported outcomes for mother-to-child HIV transmission.', 'Perinatal HIV transmission was predicted for infants of 6 weeks of age.', 'HIV-infected pregnant women who fulfill eligibility criteria are initiated on lifelong antiretroviral treatment, whereas noneligible HIV-infected women and their infants receive single-dose nevirapine in a health system functioning at reported performance levels, and the overall vertical transmission rate would be 19.5%. Adding azidothymidine for women not eligible for lifelong treatment would further decrease the overall transmission rates only marginally to 17%. If the same steps were accomplished at 95% reliability, then the overall transmission rates would be 9.4% and 4.1%, respectively.'] Are antiretroviral drugs in the cupboard enough : the impact of health systems ' performance on mother-to-child transmission of HIV?