BACKGROUND/AIMS: Retaining patients in prevention of mother-to-child transmission of HIV studies can be challenging in resource-limited settings, where high lost to follow-up rates have been reported. In this article, we describe the effectiveness of methods used to encourage retention in the Breastfeeding, Antiretrovirals, and Nutrition study and analyze factors associated with lost to follow-up in the study. METHODS: The Breastfeeding, Antiretrovirals, and Nutrition clinical trial was designed to evaluate the efficacy of three different mother-to-child HIV transmission prevention strategies. Lower than expected participant retention prompted enhanced efforts to reduce lost to follow-up during the conduct of the trial. Following study completion, we employed regression modeling to determine predictors of perfect attendance and variables associated with being lost to follow-up. RESULTS: During the study, intensive tracing efforts were initiated after the first 1686 mother-infant pairs had been enrolled, and 327 pairs were missing. Of these pairs, 60 were located and had complete data obtained. Among the 683 participants enrolling after initiation of intensive tracing efforts, the lost to follow-up rate was 3.4%. At study's end, 290 (12.2%) of the 2369 mother-infant pairs were lost to follow-up. Among successfully traced missing pairs, relocation was common and three were deceased. Log-binomial regression modeling revealed higher maternal hemoglobin and older maternal age to be significant predictors of perfect attendance. These factors and the presence of food insecurity were also significantly associated with lower rates of lost to follow-up. CONCLUSION: In this large HIV prevention trial, intensive tracing efforts centered on reaching study participants at their homes succeeded in finding a substantial proportion of lost to follow-up participants and were very effective in preventing further lost to follow-up during the remainder of the trial. The association between food insecurity and lower rates of lost to follow-up is likely related to the study's provision of nutritional support, including a family maize supplement, which may have contributed to patient retention.