A case of sacroiliitis of an 11-year-old girl with strong pain, fever and high inflammatory indices, probably caused by bacterial infection. In differential diagnosis rheumatoid process was taken into account. MR was an important diagnostic examination. After several weeks of therapy with antibiotics a great improvement in the patient’s clinical condition was achieved – regression of pain, normalization of inflammatory indices and subsidence of inflammatory changes in MR of the sacroiliac joint.