Responsively adding motivational interviewing (MI) to cognitive-behavioral therapy (CBT) for generalized anxiety disorder (GAD) has outperformed CBT alone on follow-up worry reduction , with this long-term effect on the cardinal feature of worry being mediated by less patient midtreatment resistance in MI-CBT (Constantino, Westra, Antony, & Coyne, 2019). Insofar as GAD can also be marked by interpersonal problems of nonassertiveness and over accommodation, we tested these same direct and indirect effects on these diagnostically salient interpersonal outcomes. Eightyfive patients with GAD were randomly assigned to brief MI-CBT or CBT. Patients completed a measure of interpersonal problems throughout treatment and across 12-month follow-up. Coders rated patient resistance at a midtreatment session. As expected, and consistent with the previously tested worry outcome, structural equation models showed comparable reductions in the interpersonal problems across active-phase MI-CBT and CBT. Also as predicted, MI-CBT versus CBT promoted greater reduction in over accommodation over follow-up. For problematic nonassertiveness, the effect was directionally consistent, but only approached significance. Finally, as predicted, the treatment effect on both interpersonal problem levels at 12 months following treatment was mediated by less midtreatment resistance in MI-CBT versus CBT. Results support that the benefit of adding MI to CBT for GAD extends to long-term interpersonal changes, and they implicate resistance management as a candidate mechanism of this effect.