Axial spondyloarthritis (SpA) is a chronic inflammatory rheumatic disorder that primary affects axial skeleton. It comprises wide spectrum of patients with immune mediated spine inflammation, from early, so called non-radiographic axial spondyloarthritis to clinically evident ankylosing spondylitis. Conventional radiography is still the cornerstone of diagnosis, evaluation and classification of SpA. However, it has limitations in early disease, because it can only depict the consequences of inflammation for its inability to visualize soft tissue abnormalities within bone marrow. Magnetic resonance imaging (MRI) is superior to conventional radiography in early disease through its ability to visualize active inflammatory changes in sacroiliac joints when the pelvic radiographs are normal or equivocal. MRI of sacroiliac joints is also included to the Assessment of Axial Spondyloarthritis (ASAS) classification criteria for axial SpA. For classification purposes positive definition of MRI sacroiliitis was proposed with the clear presence of subchondral bone marrow edema (osteitis), which does not cross anatomical borders and is usually present on more consecutive slides. The more intense the signal is on fluid sensitive MRI sequences; more likely it reflects active inflammation, because small focal bone marrow edema lesions may occur in patients with mechanical back pain. It may be associated with signs of structural damage such as erosions, which can enhance diagnostic utility of MRI in cases of not highly suggestive appearance of osteitis. Contrast-enhanced imaging is not useful for routine diagnostic evaluation. When MRI findings are not clear, an additional MRI of the spine can be performed, especially of the area with the most pronounced complaints. Evidence of bone marrow edema in three or more vertebral edges is considered as highly suggestive of axial SpA, especially in patients of younger age, when degenerative changes are expected to play minor role for differential diagnosis.Key words: ankylosing spondylitis - axial spondyloarthritis - magnetic resonance imaging (MRI) - sacroiliitis.