The Relational Psychotherapy Mothers' Group (RPMG), a developmentally informed, supportive psychotherapy designed to serve heroin-addicted mothers with children up to 16 years of age, aims at addressing psychosocial vulnerabilities, and facilitating optimal parenting, among at-risk mothers. We present preliminary evidence on the efficacy of RPMG as an “add on” treatment in comparison with standard methadone counseling alone. At the end of the 24-week treatment period, mothers receiving RPMG plus standard methadone counseling demonstrated lower levels of risk for child maltreatment, greater involvement with their children, and more positive psychosocial adjustment than women who received methadone counseling alone. Children of RPMG participants also reflected fewer problems in multiple areas. At 6 months posttreatment, RPMG recipients continued to be at a relative advantage, although the magnitude of group differences was often attenuated. Notably, urinalyses indicated that RPMG mothers showed greater improvements in levels of opioid use over time than comparison mothers.