The objective of this study is to investigate the use of PET-CT scan with 18F-fluorodeoxyglucose (18F-FDG) as a method to predict outcomes in patients with Takayasu arteritis (TAK), as well as to analyze associations between 18F-FDG PET-CT findings with disease relapses, sustained remission, new angiographic lesions, ischemic events, and changes in therapy for TAK. At baseline assessment, 36 TAK patients underwent 18F-FDG PET-CT scan and maximal standardized uptake value (SUVmax) in arteries ≥ 1.3 was predictive for clinical disease activity. Thirty-two TAK patients were then followed-up for a median 83.5 months. Twenty-three (71.9%) patients developed clinical relapses and new arterial lesions were observed in 14 (43.8%) cases. Disease relapses [85.0% vs. 50.0%, p = 0.049; odds ratio (OR): 5.667; 95% confidence interval (95 CI): 1.067-30.085] and the need for changing immunosuppressive therapy (85.0% vs. 41.7%, p = 0.018; OR: 7.933; 95CI: 1.478-42.581) were more frequently found in patients with SUVmax ≥ 1.3 at baseline compared with those presenting SUVmax < 1.3. No associations were found between SUVmax ≥ or < 1.3 in large arteries at baseline and the development of ischemic events, sustained remission or new angiographic lesions. In multivariate analysis, associations between baseline SUVmax ≥ 1.3 and disease relapses were not independent (hazard ratio: 1.07; 95 CI 0.39-2.92; p = 0.892). In conclusion, arterial SUVmax is marginally associated with disease relapses and with the need to change therapy in TAK. 18F-FDG uptake in large arteries is not associated with the development of new arterial lesions in TAK.