Minimizing loss to follow-up (LTFU) in long-term cohort studies is essential for reducing bias and maintaining statistical stability. However, factors associated with attrition of children in observational studies have received little attention. The authors used survival analysis methods to evaluate the association of participant and site characteristics with time to LTFU in a multicenter cohort study conducted in the United States of 2,693 human immunodeficiency virus (HIV)-infected and 1,370 HIV-exposed-but-uninfected children enrolled in 2000-2004. As of 2004, 91% of HIV-infected and 86% of uninfected children had been retained in the study. Among the HIV infected, factors associated with higher risk of LTFU included site prohibition of participant compensation, low caregiver educational level, age >15 years, and higher viral load, whereas death of a family member was associated with better retention. Among uninfected children, sites accruing low numbers of subjects, social worker responsible for retention, young age (1-2 years), and birth abnormalities were associated with higher risk of LTFU. Occurrences of certain stressful life events, such as a death in the family or financial instability, were associated with higher retention, but risk of LTFU increased when children started school or mothers began employment. Although participant characteristics are difficult to modify, the authors identified several potentially modifiable site practices that could be targeted to improve retention.