The present study simultaneously examined observed resistance and homework compliance in Cognitive Behavioural Therapy (CBT) between those with severe generalized anxiety disorder who did (n =18) and did not (n =17) receive Motivational Interviewing (MI) prior to CBT. Large effects for reduced resistance early in CBT were observed in the MI pretreatment group relative to the no-pretreatment group. Moreover, receiving four sessions of MI was associated with significantly lower levels of resistance, compared to receiving four sessions of CBTalone. Using path analysis, resistance, but not homework compliance, was found to strongly and directly mediate the relationship between treatment group and worry reduction. Findings suggest that MI improves treatment outcomes when added to CBT for generalized anxiety by reducing client resistance to, and increasing client engagement with treatment.