OBJECTIVE:: We aimed to compare the diagnostic utility of T1-weighted (T1w) and STIR MRI sequences in early spondyloarthritis (SpA) using a standardized approach to the evaluation of sacroiliac joints (SIJ) and to test whether systematic calibration of readers directed at recognition of abnormalities on T1w MRI would enhance diagnostic utility. METHODS:: Six readers independently assessed T1w and STIR MRI scans of the SIJ from 187 subjects: 75 AS and 27 preradiographic IBP patients; 26 mechanical back pain and 59 healthy volunteer controls aged </=45 years. The exercise was repeated 6 months later on a random selection of 30 AS patients and 34 controls after calibration directed at lesions visible on T1w MRI. Specific MRI lesions were recorded according to standardized definitions. In addition to deciding on the presence/absence of SpA readers were asked which MRI sequence and which type of lesion was the primary basis for their diagnostic conclusion. RESULTS:: Structural lesions were detected in 98% of AS and in 64% of IBP patients. A diagnosis of SpA was based on T1w or combined T1w/STIR sequences in 82% of AS and 41% of IBP patients. Calibration enhanced diagnostic utility of MRI in the majority of readers, especially those considered less experienced; mean (of 6 readers) pre-calibration likelihood ratio (LR) (+/-) was 14.5/0.08 and post-calibration was 22.2/0.02. CONCLUSION:: Recognition of structural lesions on T1w MRI contributes significantly to its diagnostic utility in early SpA. Rheumatologist training directed at detection of lesions visible on T1w MRI enhances diagnostic utility.