Posttraumatic Stress Disorder (PTSD) affects nearly 10% of adult women in general population samples. In populations of impoverished ethnic minority women, those lifetime prevalence rates may possibly exceed national averages due to lack of mental health resources. Mothers with PTSD are more likely to exhibit negative parenting styles and experience higher levels of perceived parenting stress, both of which are associated with poor child outcomes. However, there is a lack of evidence on how maternal PTSD may affect parenting for ethnic minority mothers. This study evaluated the prevalence of lifetime PTSD and its effects on parenting stress and infant problem behaviors in a sample of 322 low-income Mexican-American mothers (mean age = 27.8; 86% born in Mexico). Lifetime PTSD diagnoses were assessed at a prenatal home visit (24-36 weeks gestation) using the WHO Composite International Diagnostic Interview (CIDI). Mothers reported parenting hassles at 24-weeks postpartum (PDLH; Crnic & Greenberg, 1990), and child problem behaviors at infant age one-year (BITSEA; Briggs-Gowan et al., 2004). I hypothesized that 1) women with PTSD would report more parenting stress than women without PTSD, 2) women with PTSD would report more infant problem behavior symptoms than women without PTSD, and 3) parenting stress mediates the relationship between PTSD and infant problem behavior. Results found that 16.5% of women met criteria for past or present PTSD. Compared to women without PTSD, women with PTSD reported more parenting stress but a similar level of infant problem behaviors. Parenting stress significantly mediated the relationship between maternal PTSD and infant problem behaviors. Study findings suggest a need for mental health screenings during prenatal care in order to promote the healthy development of high-risk children.