Background
Metacognitive beliefs about worry may trigger anxiety. However, the effect of Generalized Anxiety Disorder (GAD) treatment on Metacognition was not yet investigated.

Aims
To validate the Metacognitions Questionnaire (MCQ-30) in a Brazilian GAD sample and verify whether different interventions reduce metacognitive beliefs.

Method
We included 180 GAD individuals randomized to Body in Mind Training (BMT), Fluoxetine (FLX) or an active control group (QoL) for 8 weeks. Internal consistency, confirmatory, exploratory factor analyses, and convergent validity with Penn State Worry Questionnaire from the MCQ-30 were applied. Generalized Estimating Equations (GEE) were conducted to evaluate differences after the interventions. 

Results
MCQ-30 demonstrated good internal consistency and acceptability; the original five-factor model was supported. There was a positive moderate correlation between MCQ-30 and worry. GEE showed a significant Group x Time interaction (p < 0.001). BMT (Mean Difference = -6.04, Standard Error = -2.39, p = 0.034) and FLX (Mean Difference = -5.78, Standard Error = 1.91, p = 0.007) decreased MCQ-30. FLX was superior to QoL but not BMT at weeks 5 and 8; there was no difference between BMT and QoL. 

Conclusion
The Brazilian-Portuguese version of MCQ-30 showed good psychometric properties. Furthermore, the positive effect of FLX and BMT on metacognition suggests it may represent a potential therapeutic target.