['To evaluate the ability of ultrasound (US) to detect the presence and change of Achilles erosions in spondyloarthritis (SpA).', 'A blind prospective two-dimensional (2D) and three-dimensional (3D) US study of Achilles enthesis erosions in early SpA was undertaken. US examinations were performed at baseline and at 6 and 12 months of follow-up. Clinical outcomes measures were collected.', 'Bilateral Achilles entheses of 68 patients (35 women) were investigated. The mean Bath Ankylosing Spondylitis Disease Activity Index and C reactive protein (CRP) levels were 4.58 ± 2.05 and 5.97 ± 9.91 mg/l, respectively. The κ values for intrareader agreement for 2D and 3D images were 0.84 and 0.85 for two readers. 2D US visualised 10 erosions (7.4%) and 3D US visualised 13 erosions (9.6%) in 10 patients (14.7%). At 6 and 12 months of follow-up, 25% and 50% of basal erosions had disappeared, respectively and, of the new erosions that appeared at 6 months, 40% had disappeared 6 months later. A statistically significant association between erosion and CRP levels, entheseal Doppler signals and the number of tender and swollen joints was found.'] Is enthesis erosion in spondyloarthritis a persistent structural lesion?