The PROMOTE study aims to measure long-term antiretroviral treatment (ART) safety and adherence; compare HIV disease progression; assess subsequent adverse pregnancy outcomes; evaluate effect of ART exposure on growth and development in HIV-exposed uninfected children; and assess long-term survival of mothers and children. This report primarily describes cohort characteristics at baseline to better understand long-term outcomes. This is a prospective study. HIV-infected mothers and their children originally recruited in a multisite randomized clinical trial for prevention of perinatal HIV transmission were re-enrolled in PROMOTE. A total of 1987 mothers and 1784 children were enrolled from eight sites in Uganda, Malawi, Zimbabwe and South Africa. Most women (≥75%) reported being married in Malawi and Zimbabwe compared to low proportions in South Africa (4.4% in Durban and 15% in Soweto), and 43.5% in Uganda (p<0.001). There were variabilities in contraceptive practices: injectable contraceptive was the commonest reported method (40.9% overall); implant was the second commonest (15.7% overall); oral contraceptives were common in Zimbabwe; and tubal ligation was common in Malawi and South Africa. At baseline, 97.8% of women reported currently using ART; 96.4% were in WHO clinical class 1 or 2; median CD4 cell count was 825 cells per uL; and viral load was undetectable in 1637 (~85%) of the women. Approximately, 14% of women did not inform their primary partners of their own HIV status, 18% reported that they knew their partners were not HIV tested, and 9% did not know if partner was tested. Overall mean age of children at enrollment was 3.5 years; and 5.7% and 25.0% had weight-for-age and height-for-age z-scores <2 standard deviations, respectively. These baseline data show high adherence to ART use. However, issues of HIV disclosure and reproductive intentions remain important. In addition to ART and ensuring high adherence, other preventive measures should be included.