Evidence for the intergenerational transmission of posttraumatic stress disorder (PTSD) is documented in the literature, although the underlying mechanisms are poorly understood. Attachment theory provides a framework for elucidating the ways in which maternal PTSD may increase offspring PTSD vulnerability. The current study utilized two independent prospective data sets to test the hypotheses that (a) maternal PTSD increases the probability of developing an insecure mother–infant attachment relationship and (b) an insecure mother–infant attachment relationship increases the risk of developing PTSD following trauma exposure in later life. In the first study of urban, primarily low-income ethnic/racial minority mothers and infants (N = 45 dyads), elevated maternal PTSD symptoms at 6 months were associated with increased risk for an insecure, particularly disorganized, mother–infant attachment relationship at 13 months. In the second birth cohort of urban, low-income mothers and children (N = 96 dyads), insecure (avoidant or resistant) attachment in infancy was associated in a dose–response manner with increased lifetime risk for a diagnosis of PTSD by adolescence. A history of disorganized attachment in infancy predicted severity of PTSD symptoms, including reexperiencing, avoidance, hyperarousal, and total symptoms, at 17.5 years. In both studies, associations between attachment and PTSD were not attributable to numerous co-occurring risk factors. The findings suggest that promoting positive mother–child relationships in early development, particularly in populations at high risk for trauma exposure, may reduce the incidence of PTSD.