Purpose:The aim is to evaluate Achilles tendon enthesopathy with ultrasound (US) in ankylosing spondylitis (AS) and non-radiographic axial SpA (nr-axSpA) patients and controls, and compare these groups in terms of associations between disease activity parameters and ultrasonographic Achilles enthesitis signs. Methods:A total of 24 AS and 20 nr-axSpA patients fulfilling the Assessment in Spondyloarthritis International Working Group criteria for axSpA and 30 controls were enrolled. Demographic characteristics, erythrocyte sedimentation rate, C-reactive protein (CRP), human leukocyte antigen (HLA)-B27, Bath AS Disease Activity Index, Bath AS Functional Index, Bath AS Metrology Index, Maastricht AS Enthesitis Score (MASES), AS Disease Activity Score-CRP, modified Stoke AS Spine Score (m-SASSS) scores and ultrasonographic findings were noted. Results: HLA-B27 positivity, extra-articular and peripheral involvement, disease activity, functional status, mean m-SASSS, ultrasonographic gray scale (GS) andtotal scores were similar between AS and nr-axSpA groups. In GS, tendon echotexture scores were significantly different across all groups (0.812 ± 0.384 in AS, 0.575 ± 0.466 in nr-axSpA, 0.017 ± 0.091 in controls; P < .001). Entheseal calcification scores were similar in AS and nr-axSpA patients, and higher than controls (P = .001).Bone profile scores were similar in patients with AS and nr-axSpA, and higher than controls (P = .010). When the correlations between US findings and disease activity and functional status were considered, power Doppler US (PDUS) and MASES total scores were positively correlated in the AS group (P = .045; r = .41). Conclusion: AS and nr-axSpA patients were found to be similar in various clinical, functional, and US findings indicating that these 2 entities are different phenotypic reflections of the same disease spectrum. The positive correlation between PDUS and MASES scores in AS patients substantiate the performance of MASES in evaluation of entheseal activity. K E Y W O R D S achilles, ankylosing spondylitis, enthesitis, non-radiographic axial spondyloarthritis, ultrasound