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Sacroiliac joint involvement is one of the earliest manifestations of psoriatic arthritis (PsA). Magnetic resonance imaging (MRI) is a useful tool in the early diagnosis of axial disease due to its sensitivity for detecting acute and chronic changes associated with sacroiliitis. In this study, we evaluated the prevalence of sacroiliitis, acute and structural image changes on MRI in PsA patients and identified predictive clinical, laboratory and disease activity factors. Cross-sectional study on PsA patients submitted to MRI of the sacroiliac joints. The scans were evaluated by two blinded radiologists and the level of agreement was calculated (kappa). Clinical, disease activity and quality-of-life indices (DAS28, BASDAI, PASI, MASES, HAQ, CRP, ESR) were estimated. The sample consisted of 45 PsA patients with a mean age of 50.1 ± 11.5 years. The prevalence of sacroiliitis was 37.8% (n = 17), 47% of which was unilateral. The kappa coefficient was 0.64. Only 5 (29.4%) of the 17 patients with sacroiliitis on MRI had back pain. The most prevalent acute and chronic changes on MRI were, respectively, subchondral bone edema (26.7%) and enthesitis (20%), periarticular erosions (26.7%) and fat metaplasia (13.3%). CRP levels were higher among sacroiliitis patients (p = 0.028), and time of psoriasis was positively associated with chronic lesions (p = 0.006). Sacroiliitis on MRI was highly prevalent in our sample of PsA patients. Raised CRP levels were significantly associated with sacroiliitis, and longer time of psoriasis was predictive of chronic sacroiliitis lesions. Most sacroiliitis patients displayed no clinical symptoms.