OBJECTIVE: Traumatic life histories are highly prevalent in people living with HIV/AIDS and predict sexual risk behaviors, medication adherence, and all-cause mortality. Yet the causal pathways explaining these relationships remain poorly understood. We sought to quantify the association of trauma with negative behavioral and health outcomes and to assess whether those associations were explained by mediation through psychosocial characteristics. METHODS: In 611 outpatient people living with HIV/AIDS, we tested whether trauma's influence on later health and behaviors was mediated by coping styles, self-efficacy, social support, trust in the medical system, recent stressful life events, mental health, and substance abuse. RESULTS: In models adjusting only for sociodemographic and transmission category confounders (estimating total effects), pasttrauma exposure was associated with 7 behavioral and health outcomes including increased odds or hazard of recent unprotected sex [odds ratio (OR) = 1.17 per each additional type of trauma, 95% confidence interval = 1.07 to 1.29], medication nonadherence (OR = 1.13, 1.02 to 1.25), hospitalizations (hazard ratio = 1.12, 1.04 to 1.22), and HIV disease progression (hazard ratio = 1.10, 0.98 to 1.23). When all hypothesized mediators were included, the associations of trauma with health care utilization outcomes were reduced by about 50%, suggesting partial mediation (eg, OR for hospitalization changed from 1.12 to 1.07), whereas point estimates for behavioral and incident health outcomes remained largely unchanged, suggesting no mediation (eg, OR for unprotected sex changed from 1.17 to 1.18). Trauma remained associated with most outcomes even after adjusting for all hypothesized psychosocial mediators. CONCLUSIONS: These data suggest that past trauma influences adult health and behaviors through pathways other than the psychosocial mediators considered in this model.