OBJECTIVE To identify factors associated with nonadherence and unsuppressed viral load across adolescence among youth with perinatally acquired HIV. DESIGN Longitudinal study at 15 US clinical sites. METHODS Self-reported antiretroviral medication nonadherence (any missed dose, past week) and unsuppressed viral load (HIV RNA > 400 copies/ml) were assessed annually. Individual, caregiver, social, and structural factors associated with nonadherence and unsuppressed viral load were identified by age (years): 8-11 (preadolescence), 12-14 (early adolescence), 15-17 (middle adolescence), and 18-22 (late adolescence/young adulthood), utilizing multivariable generalized linear mixed effects models. RESULTS During a median 3.3-year follow-up, 381 youth with perinatally acquired HIV contributed viral load measurements and 379 completed 1190 adherence evaluations. From preadolescence to late adolescence/young adulthood, prevalence of nonadherence increased from 31 to 50% (P