In previous work, Luthar and Suchman (2000, Development & Psychopathology, 12, 235) reported results of a randomized clinical trial testing the efficacy of the Relational Psychotherapy Mothers' Group (RPMG) for methadone-maintained mothers. In this extension, we examined maternal interpersonal maladjustment as a predictor of differential response to RPMG versus standard drug counseling (DC). We predicted that RPMG mothers with high levels of interpersonal maladjustment would improve on parent-child relationship indices, whereas DC mothers with high levels of interpersonal maladjustment would show no improvement. Fifty-two mothers enrolled in the study completed baseline, post-treatment and 6-month followup assessments and a subset of 24 "target" children between the ages of 7 and 16 completed measures on mothers' parenting. As predicted, results of hierarchical regression analyses indicated moderate interpersonal maladjustment x treatment interaction effects for all parenting outcomes at post-treatment and for a subset of outcomes at followup. Plotted interactions confirmed predictions that, as maternal interpersonal maladjustment increased, parenting problems improved for RPMG mothers and remained the same or worsened for DC mothers. Results indicate the potential value of interpersonally oriented interventions for substance-abusing mothers and their children.