OBJECTIVE:: To systematically assess the diagnostic utility of MRI to differentiate spondyloarthritis (SpA) patients from patients with non-specific back pain (NSBP) and healthy volunteers using a standardized evaluation of MR images of the sacroiliac joints (SIJ). METHODS:: Five readers blinded to patient and diagnosis independently assessed MRI scans (T1-weighted and STIR sequences) of the SIJ from 187 subjects: 75 patients with AS (symptom duration </=10 years); 27 patients with pre-radiographic inflammatory back pain (IBP) (mean symptom duration 29 months); 26 NSBP and 59 healthy controls </=45 years. Bone marrow edema (BME), fat infiltration, erosion, and ankylosis were recorded according to standardized definitions using an online data entry system. We calculated sensitivity, specificity, positive (LR+) and negative (LR-) likelihood ratios for the diagnosis of SpA based on global assessment of the MRI scans. RESULTS:: Diagnostic utility was high for all 5 readers for both AS (sensitivity 0.90, specificity 0.97, LR+ 44.6) and pre-radiographic IBP patients (sensitivity 0.51, specificity 0.97, LR+ 26.0). Diagnostic utility based solely on detection of BME enhanced sensitivity (67%) for IBP patients but reduced specificity (88%); detection of erosions in addition to BME further enhanced sensitivity (81%) without changing specificity. A single MRI lesion of the SIJ was observed in up to 27% of control individuals. CONCLUSION:: This systematic and standardized evaluation of SIJ in SpA patients showed that MRI has much greater diagnostic utility than documented previously. We present for the first time a data driven definition of a positive MRI for SpA.