OBJECTIVE: To investigate the extent and performance of structural lesions in sacroiliac joints on MRI (MRI-SI) in patients with axial spondyloarthritis (axSpA), chronic back pain, postpartum back pain, runners and healthy subjects. METHODS: MRI-SIs in 172 subjects, including 47 patients with diagnosed axSpA fulfilling the Assessment of SpondyloArthritis international Society (ASAS) criteria, 47 patients with chronic back pain, 7 women with postpartum back pain, 24 runners and 47 healthy individuals, were scored by two well-trained readers for erosions, fatty lesions, sclerosis and ankylosis, using an adjusted Spondyloarthritis Research Consortium of Canada scoring method. In addition, cut-off values proposed by the Imaging workgroups of Leiden and the ASAS were tested. RESULTS: Of the axSpA patients, 37 (79%) had structural lesions. Most frequently reported structural lesions were erosions in 35 (75%) patients and fatty lesions in 19 (40%) patients. Erosions and fatty lesions were uncommon in chronic back pain, runners and healthy individuals. There was a statistically significant difference in the prevalence of erosions, fatty lesions and ankylosis between axSpA and chronic back pain. Ankylosis was rare in both groups. Comparing axSpA to non-axSpA, Leiden cut-off definitions were more specific than ASAS cut-offs. CONCLUSIONS: Structural changes resembling axSpA may occur in individuals who do not have axSpA, but at a lower rate than inflammatory changes. Erosions and fatty lesions are most specific. Using cut-off definitions for these structural lesions results in good discrimination between axSpA and non-axSpA. In axSpA, structural lesions primarily occur in concordance with ongoing inflammation.