Objective: An observational case-control study was designed to analyse the discriminative value of ultrasound (US)-detected joint effusion compared with physical examination in the assessment of sacroiliac joints (SIJ) in patients with spondyloarthropathy (SpA) with or without inflammatory back pain (IBP). Methods: 45 patients with SpA, classified according to European Spondylarthropathy Study Group criteria, and 30 healthy volunteers were studied. All patients had clinical evaluation of the SIJ, Bath ankylosing spondylitis (AS) metrology index, Bath AS disease activity index, Bath AS functional index, health assessment questionnaire, patient's and physician's visual analogue scale on global disease activity. Results: Ultrasound showed joint effusion in 38.9% of SIJ of patients with SpA and in 1.7% of SIJ of controls (p