The malleability of insecure and disorganized attachment among infants from maltreating families was investigated through a randomized preventive intervention trial. Findings from research on the effects of maltreatment on infant attachment were incorporated into the design and evaluation of the intervention. One-year-old infants from maltreating families (N = 137) and their mothers were randomly assigned to one of three intervention conditions: (a) infant-parent psychotherapy (IPP), (b) psychoeducational parenting intervention (PPI), and (c) community standard (CS) controls. A fourth group of infants from nonmaltreating families (N = 52) and their mothers served as an additional low-income normative comparison (NC) group. At baseline, mothers in the maltreatment group, relative to the nonmaltreatment group mothers, reported greater abuse and neglect in their own childhoods, more insecure relationships with their own mothers, more maladaptive parenting attitudes, more parenting stress, and lower family support, and they were observed to evince lower maternal sensitivity. Infants in the maltreatment groups had significantly higher rates of disorganized attachment than infants in the NC group. At postintervention follow-up at age 26 months, children in the IPP and PPI groups demonstrated substantial increases in secure attachment, whereas increases in secure attachment were not found for the CS and NC groups. Moreover, disorganized attachment continued to predominate in the CS group. These results were maintained when intent to treat analyses were conducted. The findings are discussed in terms of the utility of translating basic research into the design and evaluation of clinical trials, as well as the importance of preventive interventions for altering attachment organization and promoting an adaptive developmental course for infants in maltreating families.