PURPOSE: To compare the characteristic MR findings of infectious sacroiliitis (IS) and ankylosing spondylitis(AS). MATERIALS AND METHODS: We retrospectively reviewed MR findings in eight patients with IS (pyogenic in six, tuberculosis in two) confirmed by culture and clinical follow-up, and in six with AS by HLA-B27 typing. A control group of 13 asymptomatic volunteers was formed, and they underwent MRI. Findings were analysed for morphology, degree of bone erosion, and adjacent soft tissue change. CT findings of AS in four patients and IS in four were also compared to MR findings. RESULTS: MR characteristics of IS included unilaterality (100 %), abnormal cartilage signal intensity (100 %), bone marrow change (100 %), contrast enhancement (100 %), erosion (63 %), and soft tissue change (63 %). MR findings of AS showed bilaterality (67 %), abnormal cartilage signal intensity (80%), bone marrow change (80 %), erosion (80 %), contrast enhancement (44 %) and soft tissue change (10 %). CT scan showed bony sclerosis and erosion (86 %), and abnormal joint space (71 %). CONCLUSION: MR findings of sacroiliitis were loss of thin zone of a cartilage and erosions on T1-weighted image, and increased signal intensity on T2-weighted image. MRI is regarded as a useful diagnostic method where conventional diagnosis is difficult, and is able to image cartilage abnormalities directly and noninvasively. Significant differences in MR findings between IS and AS were not noted, however.