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The study rationale was to assess the performance of qualitative and semiquantitative scoring methods for (18)F-FDG PET assessment in large-vessel vasculitis. Methods: Patients with giant cell arteritis or Takayasu arteritis underwent independent clinical and imaging assessments within a prospective observational cohort. (18)F-FDG PET/CT scans were interpreted for active vasculitis by central reader assessment. Arterial (18)F-FDG uptake was scored by qualitative visual assessment using the PET vascular activity score (PETVAS) and by semiquantitative assessment using SUVs and target-to-background ratios (TBRs) relative to liver or blood activity. The performance of each scoring method was assessed by intrarater reliability using the intraclass correlation coefficient (ICC) and areas under the receiver-operating-characteristic curve, applying physician assessment of clinical disease activity and reader interpretation of vascular PET activity as independent reference standards. The Wilcoxon signed-rank test was used to analyze change in arterial (18)F-FDG uptake over time. Results: Ninety-five patients (giant cell arteritis, 52; Takayasu arteritis, 43) contributed 212 (18)F-FDG PET studies. The ICC for semiquantitative evaluation (0.99 [range, 0.98–1.00]) was greater than the ICC for qualitative evaluation (0.82 [range, 0.56–0.93]). PETVAS and target-to-background ratio metrics were more strongly associated with reader interpretation of PET activity than SUV metrics. All assessment methods were significantly associated with physician assessment of clinical disease activity, but the semiquantitative metric liver tissue-to-background ratio (TBR(Liver)) achieved the highest area under the receiver-operating-characteristic curve (0.66). Significant but weak correlations with C-reactive protein were observed for SUV metrics (r = 0.19, P < 0.01) and TBR(Liver) (r = 0.20, P < 0.01) but not for PETVAS. In response to increased treatment in 56 patients, arterial (18)F-FDG uptake was significantly reduced when measured by semiquantitative (TBR(Liver), 1.31–1.23; 6.1% change; P < 0.0001) or qualitative (PETVAS, 22–18; P < 0.0001) methods. Semiquantitative metrics provided information complementary to qualitative evaluation in cases of severe vascular inflammation. Conclusion: Both qualitative and semiquantitative methods of measuring arterial (18)F-FDG uptake are useful in assessing and monitoring vascular inflammation in large-vessel vasculitis. Compared with qualitative metrics, semiquantitative methods have superior reliability and better discriminate treatment response in cases of severe inflammation.