Objective: Although integrating motivational interviewing (MI) and cognitive–behavioral therapy (CBT) has been recommended for treating anxiety, few well-controlled tests of such integration exist. Method: In the present randomized trial for severe generalized anxiety disorder (GAD), we compared the efficacy of 15 sessions of CBT alone (N = 43) versus 4 MI sessions followed by 11 CBT sessions integrated with MI to address client resistance/ambivalence (N = 42). Clients were adults, predominantly female and Caucasian, with a high rate of diagnostic comorbidity. To control for allegiance, therapists were nested within treatment group and supervised separately by experts in the respective treatments. Results: Piecewise multilevel models revealed no between-groups differences in outcomes from pre- to posttreatment; however, there were treatment effects over the follow-up period with MI-CBT clients demonstrating a steeper rate of worry decline (γ = −0.13, p = .03) and general distress reduction (γ = −0.12, p = .01) than CBT alone clients. Also, the odds of no longer meeting GAD diagnostic criteria were ∼5 times higher at 12-months for clients receiving MI-CBT compared with CBT alone. There were also twice as many dropouts in CBT alone compared with MI-CBT (23% vs. 10%); a difference that approached significance (p = .09). The treatments were competently delivered, and intraclass correlations revealed negligible between-therapist effects on the outcomes. Conclusions: The findings support the integration of MI with CBT for severe GAD and point to the importance of training therapists in appropriate responsivity to in-session markers of resistance and ambivalence.