OBJECTIVES: To investigate the diagnostic utility of different combinations of SI joint MRI lesions for differentiating patients with axial SpA (axSpA) from other conditions with and without buttock/pelvic pain. METHODS: A prospective cross-sectional study included patients with axSpA (n = 41), patients with lumbar disc herniation (n = 25), women with (n = 46) and without (n = 14) post-partum (birth within 4-16 months) buttock/pelvic pain and cleaning assistants (n = 26), long-distance runners (n = 23) and healthy men (n = 29) without pain. Two independent readers assessed SI joint MRI lesions according to the Spondyloarthritis Research Consortium of Canada MRI definitions and pre-defined MRI lesion combinations with bone marrow oedema (BME) and fat lesions (FAT), respectively. Statistical analyses included the proportion of participants with scores above certain thresholds, sensitivity, specificity, positive and negative predictive values and likelihood ratios. RESULTS: BME adjacent to the joint space (BME@joint space) was most frequent in axSpA (63.4%), followed by women with post-partum pain (43.5%), but was present in nearly all groups. BME adjacent to fat lesions (BME@FAT) and BME adjacent to erosions (BME@erosion) were only present in axSpA patients and in women with post-partum pain, but scores ≥3 and ≥4, respectively, were only seen in axSpA patients. FAT@erosion was exclusively recorded in axSpA patients. FAT@joint space and FAT@sclerosis were present in most groups, but with higher scores in the axSpA group. CONCLUSION: BME@joint space and FAT@joint space were frequent in axSpA but also in other conditions, reducing the diagnostic utility. FAT@erosion, and BME@FAT, BME@erosion and FAT@sclerosis above certain thresholds, were exclusively seen in axSpA patients and may thus have diagnostic utility in the differentiation of axSpA from other conditions.