The authors examined pilot data from an attachment-based parenting intervention for substance-abusing mothers of toddlers (ages 12-36 months). The Mothers and Toddlers Program (MTP) is a 20-week individual therapy intervention that aims to help mothers develop more balanced representations of their children and improve their capacity for reflective functioning (i.e., recognition of the intentional nature of children's behavior). The authors hypothesized that improvement in maternal representational balance and maternal capacity for reflective functioning would correspond with improvements in maternal behavior with toddlers (e.g., sensitivity to cues, responsiveness to distress, and social-emotional growth fostering) and reduction in maternal psychiatric distress and substance abuse. Eight mothers who completed MTP showed moderate improvements in representational balance and reflective functioning, and these changes corresponded with significant improvements in maternal behaviors with toddlers. The authors also compared MTP completers and noncompleters on sociodemographic and psychosocial indexes and examined the validity of the intervention's proposed mechanisms of change. Preliminary findings support the importance of attachment mechanisms and indicate that attachment-based interventions may strengthen substance-abusing mothers' capacities to foster their toddlers' socioemotional development.