<h3>Background</h3> The MRI of SI joints, included in the ASAS criteria, is being increasingly used to detect early sacroiliitis and as a tool to measure response to treatment in clinical studies. However, evaluation of the MRI of SI joints has been considered difficult to a non-experienced observer, in which case, Sacroiliitis could be mis diagnosed or delay diagnosis and cost increase. <h3>Objectives</h3> To evaluate the concordance rate of detecting abnormalities of sacroiliac joints on MRI, between a musculoskeletal expert observer and conventional radiologist, in a group of patients with SpA <h3>Methods</h3> 66 patients with diagnosis of SpA according to the ESSG (European Study Group spondyloarthritis), who were attended to a SpA clinic from January to November of 2012 and their MRI films of SI joints and its originals reports by conventional radiologist were selected. Those images were read independently in a second time by a musculoskeletal expert observer. The agreement between two readers was analyzed through the kappa statistic. <h3>Results</h3> The agreement between the two readers (musculoskeletal expert radiologist and conventional radiologist) was poor (K 0.37), on the normal case (K 0.39), and on chronic findings (K0.31), while there was moderate agreement on the presence of acute changes (K 0.45). The agreement between a rheumatologist expert in SpA and a musculoskeletal expert radiologist observer also was analyzed, the agreement was acceptable to be useful in ascertaining acute inflammatory and chronic structural changes due to sacroiliitis (sacroilitis by X-ray K 0,5; acute changes K 0,52, chronic changes K 0,61). <h3>Conclusions</h3> Sacroiliitis can be difficult to detect in early stages of SpA. The agreement for changes due to sacroiliitis by MRI between expert and non- expert radiologist in this study was poor, while between two experts readers (Radiologist and Rheumatologist) was better. Although MRI is highly sensitive in detecting early changes according to other studies, these findings may be no detected by non-expert observers, that's why training of rheumatologists and radiologists in the evaluation of sacroiliac joints on MRI is necessary, to optimize the accuracy of the diagnostic test, avoid mis diagnostic and cost increase. <h3>Disclosure of Interest</h3> None Declared