Purpose <p>Aortic dilation represents an early pathological manifestation of aortic aneurysm formation. Due to the nonlinear nature of aortic expansion, relying solely on imaging-based diameter measurements is insufficient to fully assess the severity of pathological progression and the risk of rupture. ⁶⁸Ga-FAPI-PET/CT, which enables molecular imaging of fibroblast activation protein (FAP) during extracellular matrix remodeling, has been increasingly applied to evaluate cardiovascular disease. However, the application of ⁶⁸Ga-FAPI-PET/CT in aortic diameter has not yet been reported. This study aims to investigate whether aortic FAPI uptake is associated with aortic diameter/dilation.</p> Methods <p>This was a cross-sectional study including 50 oncology patients who underwent ⁶⁸Ga-FAPI-PET/CT. FAPI uptake, aortic diameters and calcification burden were recorded. Multivariable linear regression was used to assess the associations between FAPI uptake and aortic diameters. In addition, associations between distal aortic FAPI uptake and calcification with the diameter of adjacent proximal aortic segments were also analyzed.</p> Results <p>FAPI uptake was independently associated with abdominal aortic diameter at renal artery level (β = 2.469, 95%CI 0.481–4.458, <i>P</i> = 0.016). No association was observed between aortic segments’ FAPI uptake and calcification. Furthermore, maximum FAPI uptake (β = 2.018, 95%CI: 0.291–3.745, <i>P</i> = 0.023) and calcification burden (β = 0.000533, 95%CI: 0.000040–0.001025, <i>P</i> = 0.035) of abdominal aorta were associated with abdominal aortic diameter at renal artery level.</p> Conclusion <p>Our findings indicate that FAPI uptake is associated with aortic diameter and may provide preliminary insights into early aortic remodeling, suggesting its potential as a marker for future longitudinal studies.</p>