Prevention of mother-to-child human immunodeficiency virus (HIV) transmission (PMTCT) programs are expanding in resource-limited countries and are increasingly implemented in antenatal clinics (ANC) in which HIV sentinel surveillance is conducted. ANC sentinel surveillance data can be used to evaluate the first visit of a pregnant woman to PMTCT programs. We analyzed data from Kenya and Ethiopia, where information on PMTCT test acceptance was collected on the 2005 ANC sentinel surveillance forms. For Zimbabwe, we compared the 2005 ANC sentinel surveillance data to the PMTCT program data. ANC surveillance data allowed us to calculate the number of HIV-positive women not participating in the PMTCT program. The percentage of HIV-positive women missed by the PMTCT program was 17% in Kenya, 57% Ethiopia, and 59% Zimbabwe. The HIV prevalence among women participating in PMTCT differed from women who did not. ANC sentinel surveillance can be used to evaluate and improve the first encounter in PMTCT programs. Countries should collect PMTCT-related program data through ANC surveillance to strengthen the PMTCT program.